Improvements in Electrical Thermometers or Pyrometers.
Year: 1891

page_text: Date of Application, 25th Mar., 1891.
Complete Specification Left, 23rd Dec., 1891-Accepted, 27th Feb., 1892
PROVISIONAL SPECIFICATION.
Improvements in Electrical Thermometers or Pyrometers.
I, HUGH LONGBOURNE CALLENDAR, 2 Princes Mansions, Victoria Street,
London, Fellow of Trinity College, Cambridge, do hereby declare the nature of
this invention to be as follows:-
:―
This invention has for its object improvements in electrical thermometers or
pyrometers.
I measure the resistance of the wire forming the thermometer by the Wheat-
stone balance method as follows: The ratio arms of the balance A. L, L D consists
of equal resistance coils. The other arms of the balance contain the pyrometer P
and its leads D K, F H on one side and the compensator C and a series of
resistance coils A. G on the other. These two sides are connected by a wire E F
having a divided scale attached to it. The balance is found by putting suitable
resistances in the arm A. G. and by sliding the point of contact C along the
wire E F. When the balance is found the resistances on either side of the
point C are equal. since the ratio arms A. L, L D contain equal resistances._
The compensator C consists of two leads GC, CE, similar to the leads F H, DK
of the pyrometer but joined together at C. All four leads are similarly disposed
in the tube containing the pyrometer, so that the resistance of the compensator
is always very nearly equal to that of the pyrometer leads.
The resistance of the pyrometer coil itself is therefore equal to that of the
coils A. G. plus the difference (E C CF) of the two parts of the divided wire.
If the resistance of the pyrometer coil changes so that the contact piece must be
moved to C' to restore the balance, its change of resistance is equal to twice the
resistance of the portion of wire CC so that the resistance or temperature of the
pyrometer coil (independently of its leads) can be read off directly by suitably
graduating the scale attached to the wire E F.
For most purposes it is convenient to enclose the pyrometer coil itself and part
of its leads in a tube of glass, porcelain, iron or other material to protect it.
In order to insulate the leads from each other I make them pass through holes in
thin wads of mica or other suitable material cut so as nearly to fit the tube. This
method has the advantage of giving very perfect insulation, and of preventing
convection currents of air up and down the tube.
Dated this 25th day of March 1891.
CARPMAEL & Co.,
Agents for the Applicant.
COMPLETE SPECIFICATION.
Improvements in Electrical Thermometers or Pyrometers.
I, HUGH LONG BOURNE CALLENDAR, 2 Princes Mansions, Victoria Street,
London, Fellow of Trinity College, Cambridge, 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 aud by the following statement :—
This invention has for its object improvements in electrical thermometers or
pyrometers.
I measure the resistance of the wire forming the thermometer by the Wheat-
stone balance method as follows: The ratio arms of the balance Á L, L D in
Figure of the drawings annexed consist of equal resistance coils. The other
arms of the balance contain the pyrometer P and its leads D K, F H on one side

page_text: and the compensator C and a series of resistance coils A. G. on the other. These
two sides are connected by a wire E F having a divided scale attached to it.
The balance is found by putting suitable resistances in the arm A. G. and by
sliding the point of contact c along the wire E F. When the balance is found
the resistances on either side of the point c are equal, since the ratio arms A L, LD
contain equal resistances.
The compensator C consists of two leads G C, C E, similar to the leads F H, DK,
of the pyrometer but joined together at C. All four leads are similarly disposed
in the tube containing the pyrometer, so that the resistance of the compensator is
always very nearly equal to that of the pyrometer leads.
The resistance of the pyrometer coil itself is therefore equal to that of the
coils A. G. plus the difference (Ec CF) of the two parts of the divided wire.
If the resistance of the pyrometer coil changes so that the contact piece must be
moved to c¹ to restore the balance, its change of resistance is equal to twice the
resistance of the portion of wire ce so that the resistance or temperature of the
pyrometer coil (independently of its leads) can be read off directly by suitably
graduating the scale attached to the wire E F.
For most purposes it is convenient to enclose the pyrometer coil itself and part
of its leads in a tube of glass, porcelain, iron or other material to protect it.
In order to insulate the leads from each other I make them pass through holes in
thin wads of mica or other suitable material cut so as nearly to fit the tube. This
method has the advantage of giving very perfect insulation, and of preventing
convection currents of air up and down the tube.
Figure 2 shews the part of the apparatus containing the coil in which the
temperature is measured; a is this coil, it is of wire of pure platinum about *006
of an inch in diameter, it is wound double upon a slip of mica in such a way as to
keep the convolutions separate. At its ends the wire a is connected by welding or
otherwise with leads which in most cases may be of iron but for accurate
work at high temperature should be of platinum. Copper or silver leads should
not be used as when highly heated these metals are sufficiently volatile to injure
the platinum coil by the vapour which they emit. cc are india rubber and "silk
covered copper wires attached to the leads.
The length of the leads is such that the junctions between them and the wires c
do not hecome heated in the use of the instrument. are compensating. leads
exactly similar to the leads b, except that these wires form a loop at the fore end
where the leads are attached to the coil a. The compensating leads are
attached to wires c¹ e' similar to the wires c. c. The true leads band the com-
pensating leads are threaded through holes in a number of small mica discs d d
which keep them in place and separate the one from the other.
Fig. 2 shews one of these discs.
The whole is enclosed in a tube e which is plugged air tight at e¹.
The tube e
may be of hard glass where the temperatures for which the instrument is to be
employed do not exceed 700 C.; for higher temperatures the tube should be of
glazed porcelain or silica.
Figure 3 shews in plan the bridge apparatus by which the variation in electrical
resistance of the coil a is measured. f g h i k l m n are eight binding screws upon
a base of wood or other insulating material. and serve to couple the wires cc
of Figure 2 and y and i couple the wires c¹ c¹. The binding screws k and serve to
couple a galvanometer and the binding screws m and n couple a battery suitable to
work with the galvanometer. These being the external connections of the binding
screws, the internal connections between them are as follows: is connected with
the copper wire o and this with the row of copper surfaces pp² p³ is also
connected as shewn with the binding screw g. p' is. coupled with p² through a
resistance of say 10 units p² with p³ through a resistance of 20 units-p with p*
through 40 units, pt with p through 80 units, with p through 160 units,
p with p through 320 units, and p² with ps through 640 units. The unit I select is
the resistance of a portion of the wire o equal in length to one of the larger
2:

page_text: }}
graduations on the scale q. Between the surfaces p³, p²,-18 are screws fixed into
the base and on these screws are thumb nuts rr which can be screwed down to
clamp the two surfaces p between which they are situate and so eliminate all
appreciable resistance between them. In this way the resistance in the row pl
pp may be adjusted to anything which may be required between 0 and 1270.
i is
The binding screw is connected with the turn button or contact key.
connected with m, it is also connected with k through a resistance of about 3 Ohms.
is connected with through a like resistance, it is also connected with the turn
button t. is connected with the stud u through a resistance of about of an Ohm.
Thus when t is in contact with u, the galvanometer is shunted through this small
resistance; in this way it is protected from large currents whilst the instrument
is brought into adjustment. 1 is also connected with the wire w which runs parallel
to the wire o. m and n as already stated are battery connections and n is cou-
nected with the buttons e¹ through resistances of about 20 Oh.ns in one case and
80 Ohms in the other. When the key s is brought into contact with vor v³ the
battery is coupled in circuit but through a graduated resistance bringing the
key into contact with n the full power of the battery is applied; this is only
used in making the final adjustment, a is a slider of which the underside is shewn
in Figure 4, it can be moved along upon the wires o and w and when the
button is pressed on it makes contact with the wires by depressing the bar ³
on to them. * is a spring attached to the slider and carrying the bar a and
button x¹.
On comparing this apparatus with the diagram Figure 1 it will be seen that the
anda onnnorimataly with the point F

page_text: Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what I
claim is :-
1. An electrical thermometer or pyrometer substantially as described, in which
a coil of platinum or other suitable wire and leading wires and also compensating
wires similar to the leading wires are enclosed in a tube or case of glass porcelain
iron or other suitable material and in which the measurement of the variation with
change of temperature of the electrical resistance of the coil is unaffected by any
change which may take place in the leading wires.
2. An electrical thermometer or pyrometer in which a coil of platinum or other
suitable wire and leading wires are enclosed in a tube of glass porcelain iron or
other suitable material and are kept in place by threading them through holes in
discs of mica or other suitable insulating and sufficiently refractory material loosely
fitting the said tube as herein described.
3. An electrical thermometer or pyrometer substantially as described.
Dated this 21st day of December 1891
HUGH L. CALLENDAR.