in the Improvements in Wheatstone Bridge Apparatus and Adaptation of the same to the Measurement of Temperatures, Electric Potential and Current.
Year: 1891

page_text: Date of Application, 21st Dec., 1891
Complete Specification Left, 20th Sept., 1892-Accepted, 22nd Oct., 1892
PROVISIONAL SPECIFICATION.
in the
Improvements in Wheatstone Bridge Apparatus and
Adaptation of the same to the Measurement of Temperatures,
Electric Potential and Current.
I, HUGH LONG BOURNE CALLENDAR of 2 Princes' Mansions, Queen Victoria
Street in the City of Westminster, Fellow of Trinity College, Cambridge, do
hereby declare the nature of this invention to be as follows :-
In the ordinary method of measuring a resistance the deflections of the galvano-
meter vary both with the strength of the battery and with the temperature of the
galvanometer coils. 1 compensate my galvanometer in such a way that its
deflections are the same for the same change of the resistance measured, whatever
the variations in the strength of the battery and in the temperature of the
galvanometer.
I make my galvanometer with a second set of coils called the "field-coils" in
addition to the usual coils of the instrument. These field coils are set with their
axis at right angles to the other coils, and are included in the battery circuit.
They are also provided with a shunt whose resistance is adjusted so that a suitable
portion of the battery current passes through the coils. I either make the needle
of an astatic combination of magnets, or I neutralise the earth's magnetic field on
the needle by means of control-magnets. If the strength of the battery varies, the
strength of the current both in the galvanometer and field coils varies in the same
proportion so that the deflection of the needle remains constant.
In order to compensate the galvanometer for charges of temperature, which
would make its resistance vary and therefore change its sensitiveness, I make the
field coil shunt of wire such as German-silver having a smaller temperature
coefficient than the field coils. I also put in series with the galvanometer coils a
similar resistance, which is adjusted so that the increase of resistance of the
galvanometer coils due to rise of temperature is proportional to the decrease of
current in the field coils. [1fg be the resistance of the galvanometer and a that of
the wire put in series with the galvanometer coils; and if'r be the resistance of the
field-coils and s that of the shunt; the resistances a and s must be adjusted so that
the percentage increase of the resistance (g+a) is equal to that of
A galvanometer so compensated is particularly adapted for observing small
changes of resistance by the Wheatstone Bridge method, and may be used with
great advantage in connection with electrical pyrometers and voltmeters.
For use with electrical pyrometers I arrange the apparatus as follows:-
Three of the arins A.B. BC, CD of the bridge are equal resistances. The other
arm contains HK the pyrometer wire, and A.E a resistance box.
The "field coils" and shunt S of the galvanometer are included in the battery
circuit. The galvanometer coils G are inserted between A. and C. The coils
F and G are in close proximity to each other and always nearly equal in
temperature.
DK, EH are the leads of the pyrometer. The battery circuit is connected to B
and to one end of the pyrometer coil at K. Resistances are unplugged in the
arm AE till the resistance AK is nearly equal to KC. The galvanometer may be
graduated in degrees of temperature for a range of say 100 degrees, and the
resistance of the pyrometer adjusted so that the unit coil in the box is equal to the
increase of resistance of the pyrometer wire in 100° of temperature. When

page_text: the temperature rises so that the galvanometer gets to the end of its range a plug
is put in, so that the resistance in the arm AK is reduced to its previous value, and
the needle of the galvanometer starts again at the beginning of the scale. The
resistances in all four arms of the bridge being the same as before, the galvanometer
deflections represent the same change of resistance. The rise of the degrees of the
galvanometer graduations is therefore the same in any part of the scale of
temperature, and the range of temperature covered by the galvanometer may be
made to correspond to any part of the scale by suitably adjusting the resistance in
the box AE.
In order to apply such a galvanometer to indicate variations of electric pressure,
I apply the pressure or a known fraction of it in the place of the battery to the
terminals of the Wheatstone Bridge arrangement above described; but instead of
using a pyrometer of the ordinary form, I employ a fine platinum or other suitable
wire in the arm AK of the bridge, and 1 observe the changes of its resistance due
to the heating effect of the current passing through it.
In order to compensate the voltmeter for changes in the external temperature, I
make the resistance in the arm AB or DC of the bridge of a similar wire to the
voltmeter wire, of equal resistance and temperature coefficient but of larger mass
and surface, and enclose it in the same tube with the voltmeter wire so that changes
in the external conditions affect both equally. The difference in their resistances
due to the current-heating is indicated by the galvanometer, and which will
therefore show any variation in the pressure. Large changes of pressure are
measured as before by putting adjustable resistances in the circuit to balance them.
The arrangement can also be used as an ammeter by shunting the terminals with
a suitable known resistance through which the current to be measured is passed.
Dated this 21st day of December 1891.
CARPMAEL & Co,
Agents for the Applicant.
COMPLETE SPECIFICATION.
Improvements in Wheatstone Bridge Apparatus and in the
Adaptation of the same to the Measurement of Temperatures,
Electric Potential and Current.
I, HUGH LONGBOURNE CALLENDAR of 2 Princes Mansions, Victoria Street in
the City of Westminster, 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 and by the following statement :-
In the ordinary method of measuring a resistance the deflections of the galvano-
meter vary both with the strength of the battery and with the temperature of the
galvanometer coils. I compensate my galvanometer in such a way that its
deflections are the same for the same change of the resistance measured, whatever
the variations in the strength of the battery and in the temperature of the
galvanometer.
I make my galvanometer with a second set of coils called the "field-coils " in
addition to the usual coils of the instrument. These field coils are set with their
axis at right angles to the other coils, and are included in the battery circuit.
They are also provided with a shunt whose resistance is adjusted so that a suitable
portion of the battery current passes through the coils. I either make the needle
of an astatic combination of magnets, or I neutralise the earth's magnetic field on
the needle by means of control-magnets. If the strength of the battery varies,
the strength of the current both in the galvanometer and field coils varies in
the same proportion so that the deflection of the needle remains constant.
In order to compensate the galvanometer for changes of temperature, which
would make its resistance vary and therefore change its sensitiveness, I make the
field coil shunt of wire such as German-silver having a smaller temperature

page_text: similar resistance which is adjusted so that the increase of resistance of the
galvanometer coils due to rise of temperature is proportional to the decrease of
current in the field coils. [Ify be the resistance of the galvanometer and a that of
the wire put in series with the galvanometer coils; and it be the resistance of
the field coils and & that of the shunt, the resistances a and s must be adjusted so
that the percentage increase of the resistance (g+a) is equal to that of
(+).]
A galvanometer so compensated is particularly adapted for observing small
changes of resistance by the Wheatstone Bridge method, and may be used with
great advantage in connection with electrical pyrometers and voltmeters.
For use with electrical pyrometers I arrange the apparatus as is shown by
Figure 1 of the annexed drawings.
Three of the arms A.B. BC. CD of the bridge are equal resistances. The other
arm contains HK the pyrometer wire, and AE a resistance box.
The "field coils " F and shunt S of the galvanometer are included in the battery
circuit. The galvanometer coils G are inserted between A and C. The coils F
and G are in close proximity to each other and always nearly equal in
temperature.
DK. EH are the leads of the pyrometer. The battery circuit is connected to B
and to one end of the pyrometer coil at K. Resistances are unplugged in the
arm A.E till the resistance AK is nearly equal to KC. The galvanometer may be
graduated in degrees of temperature for a range of say 100 degrees, and the
resistance of the pyrometer adjusted so that the unit coil in the box is equal to the
increase of resistance of the pyrometer wire in 100° of temperature. When
the temperature rises so that the galvanometer gets to the end of its range
a plug is put in, so that the resistance in the arm AK is reduced to its previous
value, and the needle of the galvanometer starts again at the beginning of the
scale. The resistances in all four arms of the bridge being the same as before
the galvanometer deflections represent the same change of resistance. The rise
of the degrees of the galvanometer graduations is therefore the same in any part
of the scale of temperature, and the range of temperature covered by the
galvanometer may be made to correspond to any part of the scale by suitably
adjusting the resistance in the box A.E.
In order to apply such a galvanometer to indicate variations of electric pressure,
I apply the pressure or a known fraction of it in the place of the battery to the
terminals of the Wheatstone Bridge arrangement above described; but instead of
using a pyrometer of the ordinary form I employ a fine platinum or other suitable
wire in the arm AK of the bridge and I observe the changes of its resistance due
to the heating effect of the current passing through it.
In order to compensate the voltmeter for changes in the external temperature, I
make the resistance in the arm A.B. or DC of the bridge of a similar wire to the
voltmeter wire of equal resistance and temperature coefficient but of larger mass
and surface, and enclose it in the same tube with the voltmeter wire so that changes
in the external conditions affect both equally. The difference in their resistances
due to the current heating is indicated by the galvanometer, and which will
therefore show any variation in the pressure. Large changes of pressure are
measured as before by putting adjustable resistances in the circuit to balance
them.
The arrangement can also be used as an ammeter by shunting the terminals with
a suitable known resistance through which the current to be measured is passed.
Figure 2 of the drawings shows an electrical pyrometer suitable for use in the
arts in measuring temperatures of gases or molten masses up to 1500° C.
a is a pyrometer tube of hard glass or porcelain containing near its extremity
the coil of platinum wire which has to be brought to the temperature to be
measured. This pyrometer tube is fitted in all respects as is described in the

page_text: Specification of my former Patent No. 5342 in the year 1891; it contains the
leads to the platinum coil and also compensating leads as that Specification
explains.
These four leads are coupled by the binding screws 16 3 64 with the box c in
which are contained the resistance coils and galvanometer of the measuring
apparatus. d d are binding screws for connecting the battery. e is a key for
closing the battery circuit. f is a long index upon the axis of the galvanometer
needle it moves over an are which is graduated from 0 to 100. The apparatus is
so adjusted that each of these graduations corresponds to one degree centigrade.
The galvanometer bobbins are of the ordinary oval form, indeed the galvanometer
itself presents no peculiarity except the addition to it of field coils. The field coils
are wound upon two bobbins which surround the galvanometer bobbins and are
symmetrically disposed in relation to the axis of the needle which is an astatic
combination. Upon the top of the box are brass contact pieces marked 0 to 15
they surround a central disc and there is a plug g which can be inserted between
the central disc and any one of the contact pieces around it.
There are
resistance coils within the box connecting the circle of contact pieces the one
with the other. The connections within the box are such that when the plug g is
at O all these resistances are connected in series with the platinum pyrometer coil
and this series forms one side of the Wheatstone Bridge. If the plug g be inserted
at 1 one of the set of coils is put out of circuit; if at 2 two, and so on.
I so
adjust the instrument that the resistance of one of these coils is equal to the
increase in resistance of the platinum pyrometer coil when its temperature is raised
by 100° C.
The box also contains resistance coils forming the three permanent sides of the
Wheatstone Bridge.
The connections are such as the diagram Figure 1 indicates.
In a pyrometer such as is represented in Figure 2 the following proportions or
thereabouts will be found suitable, but they admit of much variation.
Galvanometer coils.
Resistance 10 ohms. No. of turns 200.
Field coils.
Resistance 100 ohms.
No. of turns 1000.
German silver shunt on field coils.
Resistance about 10 ohms.
German silver in series with galvanometer.
Resistance about 1 ohm.
Permanent sides of bridge.
Resistance 20 ohms in each.
Platinum coil.
Resistance about 4 ohms (cold).
Plug resistances.
One ohm in each coil.
Battery, a large leclanché or storage cell.
The resistance coils forming the sides of the Wheatstone Bridge should be made
of an alloy which does not change appreciably in resistance at ordinary
temperatures.
In the use of the instrument the plug g is inserted into one or other of the holes
in accordance with the temperature expected to be found; for example if 900 C
be expected then at 9. This being done the finger key e is depressed, care being
taken however previously to observe that a sufficient length of the pyrometer tube
is fully heated to the temperature which it is required to measure. If on closing
the circuit the index passes off the scale it is an indication that the plug g should
be shifted either one step higher if the index traverses the whole scale or one step
lower if it falls away from it. When the index stops on the scale it gives the
indication required; say for example the plug g is at 9 and the index stops at 30
הסם.
Battery, a large leclanché or storage cell.
The resistance coils forming the sides of the Wheatstone Bridge should be made
of an alloy which does not change appreciably in resistance at ordinary
temperatures.
In the use of the instrument the plug g is inserted into one or other of the holes
in accordance with the temperature expected to be found; for example if 900 C
be expected then at 9. This being done the finger key e is depressed, care being
taken however previously to observe that a sufficient length of the pyrometer tube
is fully heated to the temperature which it is required to measure. If on closing
the circuit the index passes off the scale it is an indication that the plug g should
be shifted either one step higher if the index traverses the whole scale or one step
lower if it falls away from it. When the index stops on the scale it gives the
indication required; say for example the plug g is at 9 and the index stops at 30
the temperature is 030 C.

page_text: For purposes requiring the greatest attainable accuracy I use a mirror
galvanometer but in other cases the index as shown in the drawings is preferable.
In like manner I can construct a voltmeter. The points between which the
difference in potential is to be measured replaces the battery used with the
pyrometer and the pyrometer tube shown in Figure 2 is replaced by a platinum
wire so proportioned that it carries a comparatively small current and becomes
heated in so doing but not so highly heated as to risk injury to the instrument.
This wire I preferably enclose in a glass tube and I exhaust the air therefrom.
With this wire I enclose also four other similar wires.
The first wire forms together with the plug resistances one side of the
Wheatstone Bridge. Another side is formed by the other wires contained in the
glass tube; these are so coupled in parallel and series as to have when cold the same
resistance as the single wire.
The remaining sides of the bridge are other equal resistances. With the
exception of the platinum wire all the resistances are such that they do not become
appreciably heated in the use of the instrument. The proportions and the
graduations should be such that the scale may be red in volts.
In like manner an ampére meter may be constructed, the proportions given to
the parts being suitably adjusted to the current which may be passed through the
instrument.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed I declare that what I
claim is :-
1. A pyrometer substantially as described in which the electrical resistance of
a heated wire is compared with standard resistances by the aid of a galvamometer
compensated by field coils and otherwise as herein stated so that the defection of
galvanometer may be read in degrees of the centigrade scale or in other units.
2. An instrument substantially as described by which an electrical resistance
may be measured by comparison with a standard and fractional parts indicated
upon a scale by the deflection of a galvanometer which in order that it may serve
this purpose is compensated by field coils, and other appliances as herein stated.
3. The combination with Wheatstone Bridge apparatus of field coils disposed as
herein described in such relation to the galvanometer as to compensate the same so
that the indications of the instrument as read from a scale in connection with the
galvanometer may not be affected by variation in the electro motive force of the
battery.
Dated this 13th day of September 1892
HUGH L. CALLENDAR.