Improvements in the Methods of employing Materials that are Sensitive to Radiant Energy such as Light.
Year: 1891

page_text: Date of Application, 2nd July, 1891
Complete Specification Left, 4th Apr., 1892-Accepted, 2nd July, 1892
Improvements in the Methods of employing Materials that are
Sensitive to Radiant Energy such as Light.
I EUSTACE THOMAS, of 18 Sisters Avenue, Wandsworth, S.W. Electrical
Engineer, do hereby declare the nature of this invention to be as follows:-
Photometers involving the employment of materials that are sensitive to light,
such as selenium, have not proved successful up to the present, owing mainly to
spontaneous alterations in the materials themselves.
To overcome this difficulty, the applicant causes such a sensitive arrangement,
to be subjected alternately, & in rapid succession, to the light to be tested, & to a
standard light. The mean effect while facing one light is compared with that
produced while facing the other.
The best known sensitive apparatus is a selenium cell. One method of carrying
out this invention with such a cell is to mount it on an arbor capable of being
rapidly rotated. Whilst facing one light contact is made so that a current flows
through a galvanometer for a small portion of the rotation. When, in the course
of its movement, the cell faces towards the other light, contact is again made for
an equal fraction of the rotation, but the current now flows through the
galvanometer in the opposite direction, the reversal being automatically effected by
a commutator connected to the apparatus. This is repeated many times a second.
If the cell is equally illuminated by the two lights, the galvanometer needle does
not move from zero; but if one lights the cell more than the other, the needle will
move, & only return to zero when the two effects are equal. The stronger light
may be moved farther off till balance is obtained, & the relative power of the
two lights is then known by the law of inverse squares.
The method has the advantage of being a zero one, & may therefore be made
very sensitive.
The adjustment may also be got by altering the relative duration of the contacts,
by passing the currents through separate galvanometers, or the two coils of a
differential galvanometer, & then shunting one of them or adding resistance to its
circuit, or in any other convenient way.
One method of causing the current to be completed for exactly similar fractions
of the two active periods of a rotation is to cause a contact piece to revolve at
double the rate at which the cell revolves, so that the circuit is completed by the
same contact piece when in the same positions, in both periods.
The sensitive cell.may be fixed & subjected at proper intervals to the two
radiating bodies, by means of moving mirrors, screens, diaphragms &c.
The improved method of using these sensitive materials is applicable to other
purposes, such as the detection of a light-or of any other kind of radiant energy to
which the cell is-sensitive-or of its extinction.
Dated this Thirtieth day of June 1891.
EUSTACE THOMAS.

page_text: COMPLETE SPECIFICATION.
Improvements in the Methods of employing Materials that are
Sensitive to Radiant Energy such as Light.
1, EUSTACE THOMAS, of 18 Sisters Avenue, Wandsworth, London, S. W. in the
County of Surrey, Electrical Engineer, do hereby declare the nature of this
invention and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement:-
The practical employment of selenium cells for photometry has been hitherto
prevented by the spontaneous alterations in resistance to which such cells are
subject.
This difficulty may however be overcome by causing the cell to be subjected, in
rapid alternation, to the light to be tested & to a standard light, employing suitable
means to compare the effects produced.
The simplest method of carrying out this device is to mount the cell on a
spindle S Fig. 1. upon the plate P, & by causing the whole to revolve to make the
cell face periodically in the two directions desired. The current enters the cell by
the ring & wire c₁, flows thro' the galvanometer by the commutator 2 13 &
brushes & finally leaves by the brush . On the same spindle is mounted a
toothed wheel gearing with a second w, of one half the diameter & number of
teeth. Upon the latter is attached a make & break arrangement. As long as the
brush touches the circuit is complete & current goes thro' the galvanometer.
But when touches only the circuit is broken. As w₂ makes two revolutions for
each one made by w, the circuit is closed twice in each revolution of the selenium
cell for precisely equal intervals of time. 201 & we are so adjusted that the circuit
is closed when the cell faces either light the commutator 2 7'3 however causes the
current to flow in opposite directions thro' the galvanometer in the two cases.
When gear wheels are employed there is considerable noise, among other
objections during the running of the instrument to prevent this the wheels may
drive one another by friction. As however friction wheels cannot be made absolutely
true, it is necessary to take precautions to ensure their occupying always the same
relative positions at the end of each revolution of the large one.
This may be
done in many ways, such as by furnishing them with gear wheels in addition, these
wheels having however large teeth & a great deal of play with relation to one
another except in regard to one or two teeth in the circumference of the large
wheel In normal working the teeth of the gear wheels will not engage at all
except in the case of the one or two well fitting ones which will come into action in
each revolution of the large wheel & correct any inaccuracy in the relative motion
of the two friction wheels. The smaller friction wheel may be driven directly from
the larger one as in the figure, or thro' one or more smaller wheels pressing against
both in the well known way.
The galvanometer employed is preferably a sensitive reflecting one of many turns
of wire.
I find it convenient to make the shaft S a continuation of that of a small electro-
motor, which, with the addition if necessary of a flywheel, gives a very steady
result. Any other suitable method of driving may of course be employed.
If the illuminations of the cell are equal when facing the standard light & the
other, the galvanometer needle will receive equal impulses in opposite directions &
will not therefore move from zero. This condition of equal illuminations is arrived
at by any of the methods employed in ordinary photometry-most frequently by the
direct application of the inverse square law. As the method is a null one it may be
made very sensitive.
w, may be mounted on the driving shaft, so as to receive a very uniform motion,
te, being driven by it.
A second method of working is illustrated by Fio ·

page_text: But if a
Here the contact is made by a cam on the shaft which touches one or the other
of two springs h₁ h, according to the direction in which the cell faces. The
duration of the contacts can be varied by either or both of the screws a₁ a₂. The
instrument is adjusted by protecting it from light & varying a₁ or d₂ till when
revolving it does not cause the galvanometer needle to move from zero.
With an
ordinary galvanometer Fig. 2 may take the place of the wheel w, & its contact
arrangements in Fig. 1, the commutator being still retained.
differential galvanometer is employed the two coils are brought into action
alternately by contact of the cam with h₁ & h₂. Special precautions have to be taken
against play in the bearings of the spindle, such as making the cam large so as to
make such play relatively small; or mounting the spindle in bearings that admit of
easy & accurate adjustment; or mounting the cam on a second carefully supported
shaft driven from the first, or driving the first.
Instead of rotating the cell itself, a mirror or mirrors or other optical device may
be rotated in such a manner as to subject the cell alternately to the two sources of
radiation. A simple example of this method of working is illustrated in Fig. 3.
A mirror m is mounted at an angle of about 45° upon a spindle s-shown vertical
in this case.
In its present position the radiation from the candle is reflected on to
the light-sensitive cell S which is protected from other illumination than that
reaching it by the mirror by the box B; but as m revolves from its present
position the candle will cease to illumine the cell, which will then begin to receive
light from the lamp E. The contact arrangements are substantially the same as
before, e.g. there are a make & break device & a commutator to reverse the current
thro' the galvanometer.
Suitable optical arrangements may also be made for cutting off the radiation
from the two sources alternately by means of a moving screen. A device for this
purpose is illustrated in Fig. 4. The screen C is a cylinder, cut away at parts as
shown, & made to revolve uniformly about the axis o. o. In its present position
the light-sensitive cell is screened from. the candle, but exposed to the light from
the lamp.
It is obvious that instead of a galvanometer a high resistance relay may be used
which may turn on a current in a local circuit & thus cause other effects to be
produced which would be impossible with the original small currents passed thro'
the sensitive cell. Thus if a selenium cell fitted up in any of the ways described
above is placed between two lights-such as two signal lights on a railway-the
extinction of either will cause the tongue of the relay to move to one side or the
other & turn on a local circuit containing an électric bell. Light-sensitive cells
can be put in this way to many uses which have not hitherto been other than
experimental or illustrative on account of the spontaneous variations before
alluded to.
In carrying my invention into effect I do not confine myself to the forms of
contact arrangement &c. above described. They are however typical & illustrative.
Moreover, although as far as is at present known selenium cells are the best for
employment with my invention, any others will be used which may appear better
under the given conditions. It is my intention to try to obtain light sensitive cells
of quite other materials than those employed up to the present, in the hope of
obtaining greater durability & sensitiveness.
Having now particularly described & ascertained the nature of my said inven-
tion & in
what manner the same is to be performed, I declare that what I
claim is:
1. The comparison of light or other radiant energy by subjecting a cell, sensitive
to such radiation, alternately & in rapid succession to the two sources, and
comparing the two distinct effects thus produced by suitable electrical & mechanical
arrangements.
2. In apparatus for measuring radiant energy the comparison of two electrical
effects by causing them to act alternately, in rapid succession, & in opposing

page_text: directions on any instrument capable of balancing such effects & indicating the
preponderance of one over the other such as an ordinary or differential galvanometer,
or relay.
3. For the above purposes the method of closing an electric circuit twice in the
revolution of a given spindle for the same period of time- by mounting the circuit
closing device upon a second spindle caused to revolve in any suitable manner at
twice the rate of the first.
4. The arrangements for the above purposes as illustrated & described with
reference to Fig. 1.
5. The circuit closing arrangements for the above purposes as illustrated &
described with reference to Fig. 2.
Dated this Fourth day of April 1892
EUSTACE THOMAS.