IMPROVED Apparatus for INDICATING THE INTENSITY OF RADIATION.”
Year: 1875

page_text: LETTERS PATENT to William Crookes, of Mornington Road, in the
· County of Middlesex, F.R.S., for the Invention of "IMPROVED
Apparatus for INDICATING THE INTENSITY OF RADIATION.”
I, WILLIAM CROOKES, of Mornington Road, in the County of
Middlesex, F.R.S., do hereby declare the nature of the said Invention
for "IMPROVED APPARATUS FOR INDICATING THE INTENSITY OF RADIATION," to be
as follows:-.
This Invention relates to a method of obtaining mechanical movement
by means of the rays of light, heat, or actinism (for shortness included
in the term "radiation"). When "radiation," e.g., from the sun, a
candle, or diffused daylight falls upon a surface (suspended or balanced
in such a manner as to be free to move), such surface is repelled.
The
A

page_text: lighter the body, and the greater the extent of surface it exposes to the
action of radiation, the greater is the amount of repulsion, and conse-
quent motion of the substance. Thus a thin surface of pith is one of
the best substances to use for this purpose..
The color and mechanical condition of the surface have much to do
with the amount of repulsion, and consequent extent of motion pro-
duced by radiation. A lampblacked surface of pith is much more
strongly acted on than a white surface. As a rule dark surfaces are
more repelled than light surfaces, but there are many exceptions to this
rule.
Except in cases where special arrangements are employed to detect
the movement of repulsion the presence of air, gas, or vapour round
and in contact with the thin moveable surface either greatly interferes
with or entirely neutralises the repulsion by radiation; indeed, in most
ordinary cases, if air be contained in the apparatus it will by air currents,
and other less obvious actions, cause the effect of radiation to be one of
attraction. If however the instrument be enclosed in glass sufficiently
strong to stand the atmospheric pressure when air is exhausted there-
from, and the apparatus be then connected with an air pump or other
apparatus for removing the contained air, gas, or vapour, the first action
of radiation will be seen to diminish as the exhaustion proceeds, until at
some intermediate stage of rarefaction a point of partial or complete
neutrality is reached at which radiation produces no movement in the
instrument. As the exhaustion proceeds after this point is reached
repulsion is seen to be produced under the influence of radiation. This
becomes stronger as the exhaustion gets nearer perfection, the instru-.
ment being most sensitive at the highest attainable point of rare-
faction.
By suspending a thin surface of pith, paper, aluminium, or other
light material with the surface suitably prepared with lampblack, or
otherwise, to the end of a. torsion thread of glass, the amount which the
surface is repelled by radiation is in proportion to the amount of
radiation falling upon the surface, therefore by adopting any of the
known means of measuring the movement the amount of radiation
falling on the surface can be estimated with accuracy.
If a long surface of some light material, such as a rectangular bar of
pith be suspended in a long tube (with a bulb, at the ends) by means of

page_text: a silk fibre, and the air be removed from the bulb tube the bar will
rotate if radiation be allowed to fall on one extremity of it. It will also
rotate if alternate halves are coated with lampblack, and the whole be
exposed to radiation. This form of apparatus can also be used for
measuring the intensity of radiation by affixing a small magnet to the
pith, and placing a larger magnet outside the instrument so as to control
the movement and bring the pith bar back to zero when radiation ceases
to fall on it.
Another form of apparatus consists of four horizontal, arms formed of
some light material such as straw, thin glass fibre, or thin metal sus-
pended on a hard steel point resting in a jewel or class cup, so that the
arms are able to revolve horizontally upon the centre pivot. To the
extremity of each arm is fastened a thin disc of pith, paper, metal foil,
or other substance, white on one side and black on the other, so that as
the arms revolve two black and two white surfaces are always exposed.
The whole is enclosed in a thin glass globe, which is 'then exhausted to
the highest attainable point, and hermetically sealed. When radiation
falls
upon this instrument the two black surfaces always exposed on one
side being repelled more strongly, and the two white surfaces exposed
on the other side, rotation is produced, and the arms revolve with more
or less velocity, the rapidity of revolution being directly proportioned to
the intensity of the incident rays. If no screen is in front, counting the
revolutions per minute will give a measure of the total radiation. If
screens are placed in front of the instruments, such as water, alum, or
colored glass, whereby a portion only of the incident rays are allowed to
fall upon the movable discs, the instrument may be used to measure the
intensity of that portion of radiation which passes through; thus, these
instruments may be used to measure radiant heat, light (white or
colored), or actinism, the movement being produced by any portion of
the solar spectrum, although it is strongest at the red and ultra red
end.
I do not confine myself to any particular material for forming the
surface on which radiation is to act, nor do I confine myself to any
special color or preparation of the surface, or to any method of sus-
pending or balancing it, or to any form of glass case, whether cemented
together or made in one piece, nor to any method of producing or
retaining the necessary degree of exhaustion in the instrument, although
I have specified certain forms and constructions of instrument which

page_text: experience has shewn me to be most convenient. What I do claim is,
the production of an instrument whereby the intensity or amount of
radiation can be shewn by the mechanical movement produced by the
force which it exerts when falling on the surface of a body which is free
to move in a rarefied space.
SPECIFICATION in pursuance of the conditions of the Letters Patent,
filed by the said William Crookes in the Great Seal Patent
Office on the 5th May 1876.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, WILLIAM
CROOKES, of Mornington Road, in the County of Middlesex, F.R.S.,
send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Fifth day of November, in the
year of our Lord One thousand eight hundred and seventy-five, in
the thirty-ninth year of Her reign, did, for Herself, Her heirs and
successors, give and grant unto me, the said William Crookes, Her
special licence that I, the said William Crookes, my executors,
administrators, and assigns, or such others as I, the said William
Crookes, my executors, administrators, and assigns, should at any
time agree with, and no others, from time to time and at all times
thereafter during the term therein expressed, should and lawfully
might make, use, exercise, and vend, within the United Kingdom of
Great Britain and Ireland, the Channel Islands, and Isle of Man, an
Invention for "IMPROVED APPARATUS FOR INDICATING THE INTENSITY OF RADIA-
TION," upon the condition (amongst others) that I, the said William
Crookes, my executors or administrators, by an instrument in writing
under my, or their, or one of their hands and seals, should parti-
cularly describe and ascertain the nature of the said Invention, and
in what manner the same was to be performed, and cause the
same to be filed in the Great Seal Patent Office within six calendar
months next and immediately after the date of the said Letters
Patent.

page_text: same is to be performed, to be particularly described and ascertained
in and by the following statement, reference being had to the
Drawing hereunto annexed, and to the letters and figures marked
thereon (that is to say) :—
This Invention relates to a method of obtaining mechanical movement
by means of the rays of light, heat, or actinism (for shortness included
in the term radiation). When "radiation " e. g. from the sun, a candle,
or diffused daylight falls upon a surface which is suspended or balanced
in a rarefied space in such a manner as to be free to move, such surface
is repelled or moves away from the source of radiation. The lighter the
body the greater will be the extent of surface it exposes to the action of
radiation, and the worse it conducts heat the greater will be the amount
of repulsion and consequent motion of the substance; thus a thin surface
of pith is one of the best substances to use for this purpose, but I do not
confine myself to this substance.
The colour and mechanical condition of the surface have much to do
with the amount of repulsion and consequent extent of motion produced
by radiation, for instance, a lampblacked surface of pith is much more
strongly acted on than a white surface, and as a rule dark surfaces are
more repelled than light surfaces, but there are many exceptions to this
rule.
Except in cases where special arrangements are employed to detect
the movement of repulsion the presence of air, gas, or vapour round and
in contact with the thin movable surface either greatly interferes with
or entirely neutralises the repulsion by radiation. Indeed in most
ordinary cases if air be contained in the apparatus it will (by air currents
and other less obvious actions) cause the effect of radiation to appear to
be one of attraction. If however the instrument be enclosed in glass
sufficiently strong to stand the atmospheric pressure when the air is
exhausted therefrom, and the apparatus be then connected with an air
pump or other apparatus for removing the contained air, gas, or vapour,
the apparent attraction will be seen to diminish as the exhaustion
proceeds, until at some intermediate stage of rarefaction a point of
partial or complete neutrality is reached, at which radiation produces
little or no movement in the instrument. As the exhaustion proceeds
(after this point is reached) repulsion is seen to be produced under the
incluence of radiation. I have found this action to become stronger as

page_text: the exhaustion gets nearer perfection, the instrument being most
sensitive at the highest point of rarefaction I have hitherto obtained.
By attaching a thin surface of pith, paper, clear or roasted mica,
aluminium, or other light material (with the surface suitably prepared
with lampblack or otherwise) to one end of a light beam, and suspending
this to the end of a bifilar suspender of silk, or to a torsion thread of
glass or other material, the amount which the surface is repelled by
radiation is in proportion to the amount of radiation falling upon the
surface, therefore by adopting any of the known means of measuring the
movement the amount of radiation falling on the surface can be estimated
with more or less accuracy.
•
If a long surface of some light material such as a rectangular bar of
pith be suspended in a long tube (with a bulb at the end) by means of a
silk fibre, and the air be removed from the bulb tube, the bar will move
round if radiation be allowed to fall on one extremity of it. It will also
rotate if alternate halves are coated with lampblack and the whole be
exposed to radiation. This form of apparatus can also be used for
measuring the intensity of radiation by affixing a small magnet to the
pith, and placing a large magnet outside the instrument so as to control
the movement and bring the pith bar back to zero when radiation ceases
to fall on it.
I prefer to use pith in constructing such an instrument, as it does not
allow heat to pass readily from one surface to another. If instead of
pith a substance is used which permits heat to pass readily through it,
either by conduction or transmission anomalous results are sometimes
produced, and the instrument will take a long time to return to zero.
Another form of apparatus consists of four horizontal arms formed of
some light material, such as straw, thin glass, fibre, or thin metal,
suspended on a hard steel or other point resting in a jewel or glass cup,
so that the arms are able to revolve horizontally upon the centre pivot.
To the extremity of each arm is fastened a thin vane of pith,.paper,
metal foil, tale, or other suitable substance, which should be white on
one side and black on the other, so that as the arms revolve two black
and two white, surfaces are always exposed. The whole is enclosed in a
thin glass globe or bulb which is then to be exhausted to a very high
point and hermetically sealed. When radiation falls upon this instru-
ment the two black surfaces always exposed on one side. being repelled

page_text: more strongly than the two white surfaces exposed on the other side,
rotation is produced, and the arms revolve with more or less velocity,
the rapidity of revolution being directly proportioned to the intensity of
the incident rays, all other conditions (such as material and weight of
parts, colour, friction of rubbing surfaces, amount of exhaustion) being
equal. If no screen be in front counting the revolutions per minute of
the rotating vanes will give an approximate measure of the total
radiation. If screens, such as water, alum, or coloured glass (whereby
a portion only of the incident rays are allowed to fall upon the moveable
vanes) are placed in front of the instrument, it may be used to measure
the intensity of that portion of radiation which passes through these
screens, thus these instruments may be used to measure radiant heat
light (white or coloured) or actinism, the movement being produced by
any portion of the solar spectrum, although it is strongest under the
influence of the rays at the red and ultra red end.
I do not confine myself to any particular material for forming the
surfaces on which radiation is to act, nor do I confine myself to any
special colour or preparation of the surface, or to any method of
suspending or balancing it or to any material or form of case whether
the parts be cemented together or made in one piece, nor to any
method of producing or retaining the necessary degree of exhaustion in
the instrument.
In the accompanying Drawings however I have shown certain forms
and constructions of instrument which have been found by experience
to be most convenient and suitable to shew the effects of radiation, and
I will now proceed to describe these various instruments, after which I
will explain the manner in which they are constructed.
A very simple form of apparatus to shew the effects of radiation is
represented at Fig. 1. It consists of a glass tube a at the lower end of.
which is a bulb 6, in which is suspended by a fibre of silk a bar c of pith
or other light material. The silk fibre is held fast at the upper end by
being connected to a plug d, but the pith bar below is free to move.
One half of the pith bar c is blackened while the other half is left white,
the other side of the bar being treated in the same manner, care being
taken that the part which is black on one side shall be white on the
other, and vice versa. Before the upper end of the tube a is closed the
air in the tube, a and bulb must be exhausted by means of any

page_text: convenient exhausting apparatus so as to produce a good vacuum. The
upper end of the tube a may then be hermetically sealed, and the
instrument is complete. Now if the instrument be supported in a
perfectly vertical position, so that neither the silk fibre nor the pith
bar c (which is suspended therefrom) will touch any part of the tube or
bulb, and if radiation be allowed to fall on the pith bar the latter will
be caused to move round horizontally at a speed corresponding to the
amount of radiation which falls on the blackened surface. I have
found that when the pith bar is set in rotation by the action of radiation,
it will not simply turn on its vertical axis, but will be inclined to deflect
or move away laterally from the vertical line, and in so doing it will be
liable to strike the sides of the glass bulb. This will stop or very
materially interfere with its rotation, and in order to prevent this
inconvenience I suspend a weight c¹ from the lower side of the pith bar
by means of a silk fibre. This weight and its suspending fibre is
enclosed in the lower glass tube a¹, and will serve to keep the pith bar
steady. The rotation of the pith bar will continue until the resistance
produced by the torsion of the silk fibre from which the pith bar is`
suspended equals the force of radiation. Whese these two forces are
balanced the pith bar will remain stationary, but if radiation be cut off
by means of a screen or otherwise the torsion of the silk fibre will have
force enough to drive the pith bar back in the opposite direction until
all the torsion of the silk fibre is taken out. If however radiation be
allowed to exert its full force the pith bar (when once the two forces
of radiation and the torsion of the silk fibre are balanced) will remain
stationary, but if the amount of radiation diminishes so as to disturb
the balance of forces the pith bar will commence to rotate in the opposite
direction until it arrives at a point where the two forces are again.
balanced. If on the contrary the amount of radiation increases the
pith bar will again rotate in the original direction. By this means the
variations in the amount of radiation will be indicated with great
accuracy.
Although the instrument just described is capable of showing some of
the phenomena connected with radiation, it is an imperfect instrument,
but it is capable of modification and improvement so as to render it useful
as a photometer.

page_text: right angles to the former Figure; Fig. 4 is a plan view of the instru-
ment arranged with its accessories to act as a photometer. In this
instrument the same end of the pith bar is blackened on both sides,
instead of the two ends being blackened on opposite sides, as in Fig. 1.
Below the pith bar c is suspended a small magnet e and a mirror f,.
(Figures 2 and 3). A large controlling magnet g is mounted outside.
the tube a in such a manner that it can be moved up or down thereon
so as to diminish or increase its sensitiveness. The apparatus thus
constructed is enclosed in a dark box or chamber lined with black
velvet, and provided with apertures for the rays of light to pass in and
out. A lamp / placed at any convenient distance is made to throw a
beam of light through a hole in the casing or box on to the mirror f, which
reflects it back on to the graduated scale h*, as indicated by dotted lines
in Fig. 4. If therefore the pith bar c be made to turn on its centre of
motion ever so slightly, the reflected beam of light from the mirror will
travel along the graduated scale a distance corresponding to the extent.
of motion of the pith bar.
In Fig. 4, i and j are two lights, one of which may be a standard
candle, and the other a gas or other burner, the amount of light from
which it is desired to measure or estimate; k, k¹, are two screens.
whereby the light from either i orj may be cut off when required.
It will be evident that if either of the screens k or k¹ be removed,
the light or radiation from the exposed candle or burner will fall on
the blackened end of the pith bar and will repel the same, and the beam
of light reflected from the mirror will travel a corresponding distance
along the graduated scale. If the screen that has been removed be.
now replaced, and the opposite one removed so as to expose the other
light, the pith bar will be repelled in the opposite direction, and if the
photometric power of the two lights be the same, the beam of light
reflected from the mirror will indicate the same degree on the scale as it.
did in the former case but on the opposite side of zero. If the
photometic power of the two lights i and j be the same, and the two
screens k and k¹ be removed simultaneously, the radiation from the two
lights being equal and acting in opposite directions, the result will be
that the pith bar will remain stationary.
'
From the above explanation it will be understood how the instru⚫
may be used for testing the power of a gas light. for instance.
mont

page_text: 4
the standard candle may be placed at a convenient distance from the
pith bar, so that the reflected beam of light from the mirror will indi-
cate, say one hundred degrees on the graduated scale. Then the candle
having been removed the gaslight to be tested is placed at such a
distance on the opposite side of the pith bar that it exactly balances the
candle, that is to say, it must be placed at such a distance from the
pith bar that when the gaslight alone shines on the pith the reflected
beam of light from the mirror will mark 100° on the opposite side of
the graduated scale. Then by measuring and squaring the two distances
I get the exact proportion between the gas and the candle, for instance,
supposing when I place a standard candle at twelve inches from the
pith bar it marks 100° on the scale, and a gas burner at thirty-two
inches from the opposite side will mark 100°; then the square of twelve
being one hundred and forty-four, and the square of thirty-two being
one thousand and twenty-four, I find that the gas burner is equal to
more than seven standard candles. When the two lights on each side
are equal no movement of the index spot of light is produced if both
screens are removed simultaneously.
It has been already mentioned that rays from any part of the spectrum
will cause the movement of repulsion in these instruments. The dark
heat rays at the ultra red part of the spectrum act especially strongly
whilst the actinic rays at the ultra violet end act very slightly but still
appreciably.
In using the instrument as a photometer or measurer of luminous
radiation, it is essential therefore that the dark heat rays and the dark
actinic rays be cut off, as otherwise erroneous indications might be pro-
duced. These rays may be practically cut off by interposing appropriate
screens between the instrument and the candle, gas, or other light to be
tested. I have found a convenient screen for this purpose to be a. cell
made of two sheets of plate glass, about half an inch apart, and con-
taining an aqueous solution of alum and of disulphate of quinine. The
alum and water practically cut off the dark heat rays and the sulphate
of quinine cuts off the ultra violet rays.
If it be desired to use the instrument to measure the intensity of any
other rays of the spectrum, this can easily be effected by interposing
screens which will only transmit the rays which are to be measured.
Thus by interposing a screen consisting of a solution of iodine in

page_text: disulphide of carbon (which only allows. dark heat rays to pass), the
instrument becomes a thermometer; and in a similar way by interposing
a screen of ammonio-sulphate of copper the higher rays of the spectrum
which are freely transmitted by this deep blue liquid may be measured.
Fig. 5 is an elevation, and Fig. 6 a plan view of another form of
instrument, which differs from that shewn at Fig. 1 inasmuch as con-
tinuous rotation is obtained so long as radiation is allowed to act on it.
On the upper end of a glass tube a is made a bulb or globe b, which
encloses the rotating arms c, c. These latter are made of glass or light
metal, such as aluminium or copper, and they carry at their extremities
vanes of pith, mica, or metal, which are blacked on one side only. These
arms it will be seen are arranged at right angles to each other, and
through their centre part is passed a fine needle point, which rests on a
small cup m at the upper end of a glass support, which is fixed by
cement or otherwise in the glass tube a. The tube a and the bulb or
globe 6 are exhausted of air by means of a Sprengel pump, or in any
other convenient manner, and then the end of the tube a is heremtically
sealed, and when the instrument is mounted in a suitable stand it is
complete and ready for use.
Fig. 7 is an elevation of another instrument, which is a modification
of the above, the object being to prevent the arms c from getting off
their centre of rotation, which may very easily take place in the instru-
ment shewn at Fig. 5. It will be seen in Fig. 7 that instead of the
rotating arms being provided with a centre pin which rests in a cup at
the top of the support (as in the former instance), they are provided
with an inverted cup which rests on a fine centre pin at the top of the
support. The top of the inverted cup enters the lower end of a vertical
tube n, fixed to the inside of the bulb or globe, and extending down nearly.
to the top of the inverted cup, so that the latter can never fall off its
centre pin. The arms, vanes, and other parts of the instrument are the
same as in the other instance.
It will be obvious that this mode of preventing the displacement of
the arms from their centre of rotation may be adapted to the mode of
mounting shewn in Fig. 5. I have shewn this at Fig. 5ª, in which it
will be seen that the fine centre pin which rests on the cup m is extended
upwards above the level of the arms, and enters the lower end of the
vertical: tubo n, secured as in Fig. 7 to the inside of the bulb.

page_text: In the instruments above described rotation is produced by radiation
falling on the blackened surfaces sideways, or in a horizontal direction,
the rotation being due to the more energetic action of radiation on
the blackened surface in comparison to what it has on a white surface.
It has been ascertained by experiment that calling the action of
radiation on lampblacked pith as 100, the action on white pith is 18,
the difference between the two being the useful amount of residual force
which is left to produce rotation.
In Fig. 8 I have shewn another form of instrument in which there
is no counteracting force of the character of the white surfaces in the
other instruments. In this instance the vanes are blackened on both
sides, and they are attached to horizontal arms, but with their faces
set at an angle to the horizon; the instrument will therefore be acted
on most energetically when radiation falls on it in a vertical direction
either from above or from below.
In constructing instruments of this class I prefer to use numerous
vanes and arms, as shewn in the Figure, instead of confining myself
to two or three arms with a corresponding number of vanes, as in the.
other instances.
The rotating arms may either be provided with a fine-pointed centre a
pin which is supported in a glass cup at the top of the central support,
as shewn at Fig. 5, or they may be attached to an inverted cup which
will rest on a fine vertical centre pin, as shewn at Fig. 7.
An obvious modification of the arrangement shewn in Fig. 8 is to
mount the rotating arms on a horizontal spindle, as shewn at Figs. 2
9 and 10, so that they may rotate like the sails of a windmill; I have
found however that this form of instrument is not only difficult to
manufacture, but is also open to several serious objections; I cannot
therefore recommend it for use, but would prefer either of the other
forms.
S:
[In the instruments above described radiation is made to cause the arms
with the blackened vanes to rotate, but it is equally possible to make
the glass envelope, consisting of the bulb 6 and glass tube a, to rotate
while the discs and arms are kept stationary. This is effected by
holding the arms steady by means of a magnet while radiation is 3
allowed to fall on the blackened discs.. An arrangement of this kind
ย

page_text: is shown at Figure 11, in which it will be seen that the instrument is
very much of the form shown at Fig. 5, the principal difference being
that a bar magnet o is attached to the arms. By placing another
magnet p in any convenient position outside the bulb the inside magnet
and with it the arms and blackened vanes will be held steady, and
prevented from moving round under the action of radiation. If now
the instrument be placed in a vessel of water so that it will float clear
of the sides, or if it be suspended in any other convenient manner, and
if radiation be allowed to fall on the blackened vanes, the bulb or
globe will have rotary motion imparted to it in a direction opposite
to that in which the arms would rotate had they been free to move.
Fig. 12 is an elevation of a photometer constructed on the principle
of the instrument just described, that is to say, with an internal and
external magnet, but with the difference that the internal magnet is
more powerful than the external one, and moreover it rotates with the
arms and blackened vanes by the action of radiation, while the glass
envelope is held steady, as in Figures 5, 6, and 7. The internal bar
magnet is seen at o, and a smaller external magnet p is in connection
with "Morse" instrument and battery.
This external magnet p is.
very delicately balanced on its centre, and as the large internal magnet o
comes round with the rotating arms one of the poles of the small
magnet is attracted and contact is made at the bottom, thereby com-
pleting an electric circuit, so that a current of electricity will pass from
the battery q to the Morse instrument q*. A ribbon of paper is drawn
by clockwork through the Morse instrument, and at each contact of the
small magnet (that is at each revolution of the radiometer) a dot is
made on the travelling ribbon of paper, and as the latter moves at an
uniform speed the dots will be close together, if the radiometer rotates
at great speed, or they will be further apart if it rotates more slowly.
The greater or less power of radiation, and therefore the greater or less
photometric value of a burner in comparison with a given standard, will
thus be indicated by the greater or less number of dots within a given
space. Therefore if radiation from a standard candle placed at a given
distance from the instrument will cause two dots to be made on every
inch of the travelling ribbon of paper, and if radiation from a gas burner
placed at the same distance from the radiometer will cause twenty dots
on the
paper when travelling at the same speed it will follow that the
gas burner will possess ten times more force of radiation than the candle,

page_text: or in other words will be equal to ten standard candles. As the magnet
in this instrument would be too strong to allow the radiometer to start
in a faint light without some initial impetus I arrange outside the
bulb b a coil of insulated copper wire s. By depressing the key t I pass
a current of electricity from the battery 9 through the coil 8, thereby
causing a sudden deflection of the magnetic needle o. The impetus
thus gained will be maintained by the action of the light upon the vanes
of the radiometer.
I have now described the various forms or the external appearance of
various forms of radiometer. It now only remains for me to explain
the manner in which they are constructed, that is to say, the manner in
which the several parts are put together, and I would here remark that
although these parts are simple in form and apparently easy to construct
the greatest care and skill is required in constructing the complete
instrument, otherwise when finished it will either not act at all or it will
act in a very imperfect and unsatisfactory manner. I have therefore
considered it necessary to very minutely explain the mode of con-
struction, so that a skilled workman by following the instructions will
have no difficulty in successfully making cither of the instruments
above shown and described.
In constructing the bar radiometer shewn at Fig. 1 a bulb is blown
at the junction of two pieces of glass tube, thus forming a globe
between two tubes a and a¹³, as shewn in the Figure. The diameter of
the bulb is about two inches, and the length of the tubes a and a¹ about
twelve and five inches respectively, but I do not intend to confine
myself to these dimensions. The bore of each tube is slightly con-
tracted at a² and a³, and the upper contraction forms a shoulder for the
support of a plug, from which the silk fibre is suspended. The plug
which may be of a piece of tube drawn out to a point (the latter being
turned up in the shape of a hook) has an external diameter very slightly
less than the internal diameter of the tube a. The use of the lower
contraction as will be noticed further on.
A bar c of pith, mica, or other convenient material (one of the above
being the best) is cut rather less in length than the internal diameter of
the bulb b, so that when suspended therein it may revolve freely. The
width of the bar c should be as great as the contraction a² will allow to
pass. Taking pith as our example, a fine hole is pierced through the.
.

page_text: centre parallel to the broad surfaces of the bar, and one end of a long
cocoon fibre is passed twice through the hole in the same direction, thus
fixing the bar in one position on the fibre. A thin paste of very fine
lampblack (sold under the name of ivory black) mixed with alcohol is
now made. This paste is brushed on the alternate halves of the bar.c
with a camel's hair brush, and is laid on sufficiently thick to form a
dense black surface when dry. If the substance be not inflammable,
such as mica or metal, the surface may be blackened in a smoky flame,
such as that produced by the combustion of turpentine or camphor. The
barc is now ready for suspension in the bulb b. The position of the
hook of the plug at a³ (when in its place in the tube a) is marked on
the outside of the latter, and by the aid of this mark a loop may be
made in the silk at such a distance from the bar that when suspended
from the hook the bar shall be in the centre of the bulb b. A piece of
fine hard brass wire, bent in the form shown in Fig. 1', is now fastened
to the lower end of the silk fibre at such a distance below the bar c that
the wire shall just hang below the lower contraction a³. This wire
forms a kind of spring, the sides of which will contract under pressure
and expand again when released. The silk loop being fastened to the
hook of the plug above by means of a little shellac the whole is lowered
down the tube a till the shoulder of the plug rests on the contraction a².
The bar c now falls into its horizontal position in the bulb b, the little
wire spring resting on the lower contraction as in the short lower tube.
This spring is then drawn past the contraction as by means of a wire
with a hook at the end, but will not pass back again, thus by means of
its weight keeping the bar c always in the centre of the bulb b, and
thereby making the carriage of the instrument very safe. The
suspending plug above is fastened to the tube a by melting a piece
of shellac between the two; the upper end of the tube a is then drawn
off and rounded in the blow pipe flame, and the end of the lower tube a¹
is drawn down or contracted to a capillary bore a little way below the
wire spring weight, ready to be sealed off after the tubes and bulb have
been exhausted of air. The mode of effecting this object is by con-
necting the instrument to a Sprengel pump, the construction and
operation of which is well known. In addition to this I have found
that by adapting to the instrument (while being exhausted) a vessel
containing carefully prepared charcoal a large portion of the residual
air or gas will be absorbed. I do not however claim this as any part of
my Invention. _

page_text: .
In Fig. 13 a tuber of hard glass about three or four inches long is
filled with charcoal carefully prepared; this tube is drawn down at both
ends and is blown on to the end of the lower tube a¹. The tuber is
bent at right angles, as shewn in the Figure, and the instrument is
turned upside down, immersed in a vessel of water, and is attached to a
Sprengel air pump. By heating the water the instrument is kept at a
temperature of two hundred and twelve degrees Fahrenheit during the
whole time of exhaustion. When the apparatus is well exhausted the
charcoal tube is heated very strongly, thus driving off from it a large
quantity of gas which is taken out as quickly as possible by the pump.
The charcoal tube is heated two or three times, thus washing the
apparatus out (as it were) with gases that the charcoal contained; the
effect of this will be that the small portions of air left after the
exhaustion is made as complete as possible by the pump will be
absorbed by the charcoal. When the vacuum is as perfect as practicable
the instrument is sealed off with the charcoal tube still attached and put
aside for a few days, after which time the charcoal tube may be con-
sidered to have done its work, and can then be sealed off at the end of
the tube a', Fig. 13.
The mode of constructing the radiometers shewn at Figs. 2, 3, 4, 5,
5, 6, 7, and 8 is as follows:-Fine glass fibres are drawn from thicker
tubes in the blow pipe and cut into three inch lengths. Each of these
is passed through a small thin disc of pith about one-eighth inch in
diameter and the disc placed in the centre, as shewn at Fig. 14. On
each side of this pith centre the glass is bent twice at right angles in a
very tiny gas jet into the form shewn at Fig. 15; the glass arms are then
cut off to the proper lengths so that when the discs or vanes are on the
ends of the arms the distance between their further edges shall be.
about quarter inch less than the internal diameter of the bulb of the
instrument. The pith discs or vanes having been cut out with a sharp
punch (the size being regulated by the diameter of the tube on which
the bulb is blown) are struck on the ends of the glass arms by first
making a hole with a needle and then putting in the glass. All the
junctions between the glass and pith are now cemented with diamond
cement (or any other cement that will stand a temperature of a
212° Fahrenheit without softening) and are thoroughly dried. A very
fine bead needle has a little disc of paper cemented to it a little distance
from the point, the cement used in this case being shellac. The distance

page_text: *
between the point and the paper disc is arranged so that when passed
through the two little pith discs in the centre of the arms the needle
point will project sufficiently to keep the arms above and prevent them
from touching the edge of the cup in which the needle points rests.
Having placed two of the double glass arms across so that the little discs
in the centre of each come over each other the needle point is pushed
through the centre.
The
In order to allow the needle to pass through the centre the glass arms
have to be passed through slightly on one side of the centre of these
little discs. The lower disc is cemented by means of shellac to the
needle point, thus leaving the other disc free to move between the pith
disc of the lower arm and the paper disc attached to the needle.
discs or vanes at the end of the arms are now blacked on one side, the
black sides all facing one way. The part of the needle above the paper
disc is now snipped off with a pair of cutting pliers. If made accurately
with vanes of the same thickness, this little apparatus will balance on
the point without further trouble. If the vanes or arms do not balance
thin slices must be cut off the white side of those vanes which are too
heavy. In order to admit of this little instrument being passed down
the neck of the bulb, the upper or moveable glass arm must be turned
round till the two arms are close together, as shewn at Fig. 16. When
the arms have been introduced into the bulb they are opened out by
means of a couple of straws or other convenient tools until they are at
right angles to each other, as shewn at Fig. 4, the two little discs in the
centre are then cemented together with cement and dried by warning
the bulb and sucking air through by means of a tube. The supporting
cup is now blown and sealed in its place. The bulb 6 and tube a are
then to be exhausted by means of a Sprengel pump, as in the former
instance, for which purpose a charcoal tube, such as that described
previously may be sealed on to the lower end of the tube a and bent at
right angles as in the other case, and the exhaustion carried out in
exactly the same manner as with that instrument.
I would here observe that although I prefer to use the charcoal to
absorb the residual gases it is not absolutely necessary as I sometimes
dispense with its use.

page_text: the bulb until the needle point is exactly opposite the glass cup, then
suddenly turning the bulb the right way up; if done sufficiently quickly
the point invariably falls into the glass cup. This operation, however,
although apparently exceedingly simple, is considered a disadvantage,
and in order to overcome this difficulty the instrument is constructed in
such a way that the radiometer can never fall out of its position. This
is effected by leaving about one-eighth of an inch of the needle pro-
jecting above the little paper disc, and sealing to the top of the bulb
a piece of small tube a (Fig. 5"), the end of which goes over the
projecting piece of needle and prevents it from getting out of the cup.
If it be necessary to make an instrument of this kind it will be more
convenient and easier to construct if instead of the needle point the
cup is put in the centre of the moving parts of the instrument, as shewn
at Fig. 7. A fine metal point is then sealed in the upper end of the
support instead of forming a cup thereon as before mentioned. When
radiometers are made according to this last-mentioned way, it is best to
make the arms of some metal, as it is very difficult to make the little
centre pith discs swivel nicely on such a comparatively level centre as.
the glass cup would necessarily make.
The metallic armed radiometer differs from the one just described
only in the construction of the arms, to produce which two very thin
strips of metal (brass, aluminium, German silver, or other convenient
metal) are cut (with a pair of scissors) about two or three inches long,
according to the size of the instrument required, and a fine hole is
punched through the centre of each. Into one is soldered with hard
solder a fine needle, so that the point projects about 3/8 of an inch
below the strip of metal. The other strip of metal is now put over the
eye end of the needle and passed down till it rests on the one soldered
near the point. A very small disc of the same metal is then passed
down the needle in the same way and soldered to the latter, thus (as in
the case of the glass armed radiometer) leaving one strip of metal
forming two of the arms free to turn between the disc soldered to the
needle, and the lower strip of metal forming the other two arms.
arms are now bent twice at right angles, like the glass arms. This
bending is in order to bring the centre of gravity of the whole instru-
ment sufficiently below the point of suspension so as to make it easily
balanced. If a cup is put in the centre of the arms instead of a point
the arms need not be bent, as the suspending point passes above the
The

page_text: centre of gravity of the radiometer. The discs or vanes are put on in
the same way as in the other case. If mica or metal discs or vanes are
put on the arms are cut very thin at the ends and passed through a
small hole pierced in the discs or vanes a little way from the edge, then
bent back and twisted over themselves, thus holding the discs or vanes
firmly without any cement.
In order to pass the metallic arms through the narrow tube a into the
bulb 6 they are folded together, as in the other case, by moving the
moveable arm on its centre, and when opened out in the bulb the
metallic arms are fixed at right angles to each other by soldering them
at the centre by means of a small copper bit fixed in the end of a glass
tube. By this means the necessarily small piece of copper holds the
heat for some time owing to the non-conducting power of the glass.
The exhaustion of the tube a and bulb b is effected in exactly the same
manner as that described for the bar radiometer.
Radiometers of the kind just described may be made with almost any
number of discs or vanes, it being merely necessary to cut the desired
number of arms and put them on the needle between the lower fixed
strip of metal and the disc which is soldered to the needle above.
Radiometers with more than four discs or vanes can also be made
with glass arms, though when there are more than six or eight vanes at
the most, the distance between the lowest two arms and the top two
becomes too great on account of the thickness of the centre pith discs,
and the instrument will not then balance readily.
In radiometers of the class shewn at Fig. 8, there are generally the
greatest possible number of discs that can be got in the bulb, all of
which are black on both sides and are put on the arms at an angle with
the latter. An angle of 45° is a convenient one, but other angles will
do. I have made instruments of this kind with 10 or 12 vanes, although
the instrument will work with any smaller number. The methods of
support, mode of exhaustion, the shape of bulb, and other conditions
are the same as in the previously described instruments. This radio-
meter turns in different directions, according to whether the greatest
intensity of radiation be above or below the vanes. This form of
instrument can also have the moving arms mounted on a horizontal
axis instead of a vertical one, and it will then move readily to radiation
falling on it in the direction of the axis. The construction of a radio-

page_text: meter of this latter kind is however not to be recommended, owing to
the difficulty of balancing it. If one side of the instrument be at all
heavier than another, such part will fall round and become the lowest,
and much if not all of the force of radiation will be expended in
overcoming the resistance caused by gravity, and consequently the
instrument will not act in a satisfactory manner.
Having now described my Invention of "Improved Apparatus for
Indicating the Intensity of Radiation," and having explained the
manner of carrying the same into effect, I claim as the Invention
secured to me by Letters Patent as aforesaid, the instruments or
apparatus herein shown and described, whereby motion is by the action
of rays of light, heat, or actinism (denominated by me radiation) imparted
to solid substances suspended or accurately balanced in a case, vessel, or
chamber, from which the contained air or gases have been exhausted
or rarefied to a high degree, as and for the purposes herein set forth.;
In witness whereof, I, the said William Crookes, have hereunto
set my hand and seal, the Fourth day of May, in the year of
our Lord One thousand eight hundred and seventy-six.