Scale by Means of Change of Electrical Resistance.
Year: 1887

page_text: Date of Application, 25th Oct., 1887.
Complete Specification Left, 21st July, 1888
Complete Specification Accepted, 14th Sept. 18
Absolute Scale by Means of Change of Electrical Resistance.
I HUGH LONGBOURNE CALLENDAR, Fellow of Trinity College, in the University
of Cambridge, England do hereby declare the nature of this invention to be as
follows:-
My invention relates to Improvements in the practical measurement of Temperature.
The principles of the method and the experiments on which it is based need not be
explained here as they will be found fully discussed in my Paper published in the
Philosophical Transactions of the Royal Society of London Part A 1887.
The Improvements claimed as new consist
(1) In a duplex arrangement for measuring the resistance by the Wheatstone
Balance, whereby the Apparatus is rendered direct reading with a scale of
equal parts.
(2) In the use of a compound wire for the Pyrometer so constructed that the
variation of its resistance is proportional to temperature measured on the absolute
scale.
(1) The duplex arrangement aforesaid is as follows- Eeta Rhombus A B C D whose
diameters are A C, B D represent in the ordinary diagrammatic way a Wheatstone
Bridge or balance. The Battery is connected at A and C, and the galvanometer at
B and D. The opposite sides A B, C D contain similar and equal Pyrometer wires
from the variations of whose resistances their temperatures may be deduced.
The opposite sides B C, A D contain equal and similar known resistances, which are
adjusted by sliding galvanometer contacts at B and D or otherwise, till a balance is
found in the ordinary way.
If then the Pyrometer wires be wound up together, but insulated from each
other, and exposed to the temperature to be measured, their resistances and mean
temperatures remaining equal, the whole resistance of the arm A. B. C, will be equal
to the whole resistance of the arm A D C; and the resistance of the portion A B
containing one of the pyrometer wires, will be equal to the resistance of the
Rias SL

page_text: portion A D against which it is balanced, which contains only known resistances.
Thus the variations of the resistance of the pyrometer wire are directly measured by
the equal variation of the resistance required to balance it, which may be read off on
a scale of equal parts.
If we make the variation of the resistance directly proportional to the change of its
temperature measured on the absolute scale; the scale may be graduated in degrees of
temperature so that the thermometer becomes a direct reading instrument.
(2) The construction of a compound pyrometer wire such that the variation of its
resistance is proportional approximately to the variation of its temperature on the
absolute scale depends on experimental comparisons of wires of different metals such
as those described in the above mentioned paper, Philosophical Transactions 1887.
It is there shewn that the resistance variation of certain metals is expressed to a
close approximation for a considerable range by the formula
R/Ro+at+ẞt².
The sign of the coefficient ẞ is positive for some metals and negative for others.
By combining in the right proportion two metals for which the sign of ẞ is different
it is possible to eliminate this term and make the resistance variation directly
proportional to the absolute temperature-Thus for Iron ẞ is large and positive, and
for Platinum small and negative. By combining Platinum and Iron wires whose
resistances have the ratio of 1 to 076 approximately we obtain a compound wire,
whose resistance variation is very nearly proportional to the absolute temperature
between 0 and 600° C.
It may hereafter be found advisable to combine three or more wires on the same
principle to secure an even closer approximation but it is probably best to apply
small residual errors by tabulating them as for mercury thermometers at each degree
of the scale.
I 1

page_text: where a and ẞ are coefficients, constant for any particular wire.
For measuring the varying resistance of metallic wires in order to estimate
temperature, what is known as the Wheatstone Bridge 'Null' method has frequently
been employed. I also make use of this method but I modify it so that thermometric
degrees may be read off on a scale of equal parts.
I also by my invention provide for the compensation of the instruments exposed to
the temperature which has to be measured. (To avoid repetition I shall term these
instruments thermometers.")
#
When compensated thermometers are employed every equal space or degree of
the scale of my apparatus represents an equal increment of true or absolute
temperature. I attain this result by making the apparatus duplex and symmetrical
throughout.
The principle of my duplex symmetrical arrangement may be explained as
follows:
In Figure 1 of the annexed drawings let the rhombus A B C D represent
diagramatically a Wheatstone Bridge; the battery b-being connected at A and C and
the galvanometer 7 at B and D. In the opposite sides A B, DCI provide equal and
similar pyrometer wires or thermometers, from the variations of whose resistances
their temperature may be deduced.
The opposite sides B C, A D contain equal and similar known resistances, which
are adjusted till a balance is found. If then the thermometer wires be wound up
together, but insulated from each other, and exposed to the temperature to be
measured, their resistances, if they are similar wires, will remain equal the one to the
other; and the whole resistance A B C will be equal to the whole resistance A D C,
and the part A B to the part A D. Bui A B contains one of the thermometers
and A D known resistances. Thus the variations of the resistance of the
thermometer are directly measured by the equal variation of the resistance^required
to balance it.

page_text: resistance of the galvanometer contacts.
The arrangement possesses other advantages which will best appear if I describe one
of the forms of the actual apparatus.
Figure 2 of the annexed drawings shows a plan of the apparatus as arranged
for use.
The arrangements of the base board are as follows:-
On a mahogany board 4 feet by
inches are twelve plates of copper 1, 2, 3 ... 12.
Plates 1 and 6 are soldered to binding screws D D¹ for the battery wires.
Plates 1, 2, 3 . . . 10 are beneath an ebonite slab of an inch thick, they are
fixed to it and holes are bored in the ebonite to forin mercury cups, the bottoms of
which are the copper plates.
Plates 7 and 12 are connected and likewise plates 10 and 11.
B B are platinoid bridge wires stretched between plates S and 11, and 9 and 12.
along a scale of equal parts:
Sliding contacts moveable along the bridge wires connect them with. the galvano-
meter.
S, S¹ and C, C are resistance coils contained in a box. The box has eight connecting
legs and these stand in the cups 2, 3, 4, 5, 7, 8, 9, 10, making connections as
indicated.
g.g are galvanometers.
The connections will be best followed in the diagrams Figures 3 and 4.
Rand R represent the equal and similar thermometer wires, which are generally
wound up together, but insulated from each other, they are exposed to the same
temperature viz. that to be measured so that their resistances are always equal.
Similar and equal stout connecting wires of copper usually forming a sextuple cable
pass from the thermometers to the mercury cups 1, 2, 3, 4, 5, 6.
The resistance coils S, S¹ C, C are such that C+ R is nearly equal to S.
THE THERMOMETERS.
.The construction of the thermometers R, R', may be varied to suit the particular
object for which they are required.
The following is a description of some very delicate thermometers, which were used
with the bridge above described, for some physiological work at ordinary atmospheric
temperatures. The two thermometers were made exactly similar and of nearly equal
resistance (about 8 ohms at (°C), but were not wound up together, in order that it
might be possible to use them differentially, i.e. for determining small changes or
differences of temperature.
Figure 5 represents one of them, magnified ten diameters. It consists of 5.44 cm.
of very pure and fine platinum wire, 001 inch diameter, wound in eight turns on a
plate of mica about 3 mm. square and 02 min. thick.
One end of the fine wire is soldered to a double electrode of No. 36 Cu. wire, and
the other to a single electrode of the same length and material.
The thermometers have generally to be used in watery fluids, and were therefore
insulated by applying a plate of mica to either side and running in soft wax.
It is possible to make thermometers of this kind accurate to about th of a degree
Centigrade, and sensitive to about robooth C.
It is unnecessary to compensate "these thermometers when used only for small
ranges of temperature."
Resistances. In the apparatus for measuring the resistance everything is duplex.
There are two thermometers R R', variable about 8 ohms, with a temperature
coefficient of about 0033 per 1° C.
Two resistances of platinoid S, S¹ each nearly 9 ohms.
Two compensators of copper CC, each nearly 0.6 ohm.
Two bridge wires B B about 0.62 ohm.

page_text: The object of this apparently unnecessary complication is to eliminate several sources
of error, and to make the apparatus direct reading.
Referring to the diagram of connections (Figure 4) showing the Wheatstone Bridge
arrangement: we see that the resistance RCB' S₁ will be always nearly equal
to R¹ C B S.
When a balance is found by shifting the contacts at bb, we shall have R' + C + 7 b
nearly-
as before RS'− C + = + (86' 76) nearly. The 's cancel out.
=
-
If the connecting wires are equal and similar and are twisted up together so as to
be always at the same temperature, variations in their temperature are immaterial,
because they cancel each other.

page_text: correct to 0.01 %, if R and R₁ are equal to 1·0%. i.e. the mean of the resistances R. R¹
is directly given by the bridge readings, with accuracy to the second order of small
quantities, the differences of the resistances employed being small quantities of the
first order. This is a very great practical advantage of the symmetrical arrangement.
FURTHER REMARKS ON CONSTRUCTION AND MOUNTING OF THERMOMETER COILS.
To obtain the full advantage from the use of the resistance thermometer the wire
should be disposed in the way that best suits the particular experimente.g. to determine
the mean temperature of a long rod the wire should be extended along the axis of the
rod, or wound spirally round it.
°
In some cases the temperature at or near a point is required. In this case the wires
of the two thermometers are interwound in a double screw thread on mica or other
insulating material, as compactly as possible. For many experiments however it is
more convenient to have R and R' separate.
FURTHER REMARKS ON RESISTANCE OF PARTS OF APPARATUS.
This will be varied dependent on the range of temperature to be covered and the
accuracy required.
If the bridge scale be 1 metre long and each bridge wire have 1 ohm resistance
and the temperature coefficient of the pyrometer wire be 003333 per 1° C. then for a
range of 0° to 100° C. R and (SP) should be each about 6 olims (the two bridge
wires together being 2 ohms; giving a scale of 2 cm. to a degree C.).
Again with the same bridge wire for a range 0°
-
1000° C., Rand (SP) should
be each about 0-6 ohms, giving a scale of 2 mm. to a degree C. and for a range of
only ten degrees, 60 ohms; 20 cm. to a degree C.
The same bridge may be used for several different thermometers, but it is better to
use different bridge wires and special compensated coils, specially suited to each case.
Compensated thermometers. The construction of a compound pyrometer wire such
that the variation of its resistance is proportional approximately to the variation of its
temperature on the absolute scale depends on experimental comparisons of wires of
different metals such as those described in my paper published in Philosophical
Transactions 1887. It is there shown that the resistance variation of certain metals is
expressed to a close approximation for a considerable range by the formula
R/R o=1+at+ẞt. The sign of the coefficient ẞ is positive for some metals and
negative for others. By combining in the right proportion two metals for which the
sigu of ẞ is different it is possible to eliminate this term and make the resistance variation
directly proportional to the absolute temperature. Thus for iron ẞ is large and
positive, and for platinum small and negative. By combining platinum and iron wires
whose resistances have the ratio of 1 to 076 approximately we obtain a compound wire
whose resistance variation is very nearly proportional to the absolute temperature
between 0 and 600° C. It may hereafter be found advisable to combine three or more
wires on the same principle to secure an even closer approximation; but it is probably
best to apply small residual errors by tabulating them as for mercury thermometers at
each degree of the scale.

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:
-
1. The duplex symmetrical arrangement substantially as described of apparatus for
ascertaining temperature by the measurement, by the Wheatstone Bridge "null"
method, of the electrical resistance of parts (herein termed "thermometers ") exposed
to the said temperature.
2. Apparatus measuring temperature by the Wheatstone Bridge "null" method
with compensated thermometers consisting of two or more metals so combined that
the variation of resistance is proportional to the temperature measured on the absolute
or true scale.