Improvements in Electrical Thermometers
Year: 1895

page_text: Date of Application, 30th May, 1895-Accepted, 1st Feb., 1896
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We. GEORGE CHANDLER WHIPPLE, Biologist, residing on Knowles Street in
the City of Newton, County of Middlesex, State of Massachusetts, United States
of America, and HENRY ELLIS WARREN, Electrician. residing on Station Street,
in the said City of Newton, do hereby declare the nature of our invention for
Improvements in Electrical Thermometers, and in what manner the same is to be
practised, to be particularly described and ascertained in and by the following
statement :-
Our invention relates to that class of thermometers by which the temperature at
a distant or temporarily inaccessible place may be indicated or ascertained at a
convenient place of observation by means of the electrical connection of the
portions of the apparatus located at the respective places. Electrical methods and
apparatus for this purpose are of two classes, namely, varying resistance and
thermal junction. Our invention belongs to the first mentioned class, in which
heretofore the apparatus used has been defective or subject to great disadvantage
in practical use, for the reason that it showed, not the absolute temperature at the
distant point, but rather the difference between the temperature at that point and
a temperature which might be conveniently determined at the place of observation,
for instance, that of melting ice. The chief reasons which have prevented the
commercial use of these prior devices are, the expense of the apparatus, which has
been cumbersome as well as requiring a sensitive galvanometer; the necessity of
employing a skilled electrician to set up the apparatus and make computations
therefrom when used; the long time required for this, and also the expense of the
leading or line wires which must be made large to give low resistance.
By our improved apparatus and method we believe that the above mentioned
difficulties and objections have been overcome.
Our invention consists of two electric conductors of different materials which.
have different known temperature coefficients of electrical resistance and are to be
located at a distant or temporarily inaccessible point. These two materials
constitute electrical resistances, and we ascertain the different resistance ratios
between them by properly connecting them with a device for measuring ratios of
resistance, such for instance as that commonly known as the Wheatstone bridge,
or a differential galvanometer as a part of the indicating portion of our apparatus ;
and when using a Wheatstone bridge a telephone with some current varying device
may be employed instead of a galvanometer. These electrical resistances of
different materials, may be conveniently in the form of coils and for brevity we
will hereafter designate them as "temperature coils," and the device by which the
ratios of resistances between these two coils, produced by different temperatures,
is measured, we will designate as the "ratio-measurer."
The manner of determining the temperature at a distant or temporarily
inaccessible location is as follows: The temperature coils are placed near together
at that location and electrically connected with the ratio-measurer, and a battery
or other source of electrical supply. Then any ratio of resistance between the
two temperature coils which may exist at a given time can be readily determined
by the location of the movable part of the ratio-measurer upon a scale graduated
to indicate ratios of resistances between the two temperature coils, for the various
degrees of temperature within a desired range, which have been previously
determined.

page_text: In the drawings accompanying this Specification we have illustrated specific
forms of devices, which may be used to make up our improved apparatus
wherein :
Figure 1 is a diagramatic view of all the devices connected for use, with a
telephone and interrupter for indicating the presence of electric current.
Figure 2, is an elevation of the device for measuring ratios of resistance and
Figure 3, is a plan view of the same showing a pointer and graduated scale, and
a galvanometer for the current indicating device.
Referring to the drawings, X and Y are two temperature coils of different
metals, for instance X, of copper and Y of German silver, C is the electrical
conductor joining the two coils by one extremity, and L, L₁, are the leading or
line wires which connect the other extremities of said coils respectively with the
two terminals D, E, of the said ratio-measurer R, which is a modification of what
is commonly known as the Wheatstone slide-wire bridge, and consists of a hard
rubber or other nonconducting disc R', around the periphery of which the wire F,
is secured.
A movable arm H, is pivoted at I, in the centre of the disc, R'. To the outer
end of the arin H a spring contact slide K is attached, and this is arranged so as
to bear upon the wire around the periphery of the disc R. Above the disc R¹,
and concentric therewith on the pivot I, is a circular card M, graduated around its
edge to indicate degrees or fractions of degrees of temperature which correspond
to the ratios of resistances which will be produced in the temperature coils by the
various temperatures indicated. Above this card is a pointer P secured to the
pivot I, and upon the upper end of the pivot is a knurled or milled wheel N by
which the arm H and pointer P may be turned. Electric current is supplied by a
battery Q, its two poles being respectively connected to the terminals D, E, of the
ratio-measurer.
At the junction of the two resistance coils X, Y, (Fig. 1) one end of a line S is
connected and the other with the sliding contact K of the ratio-measurer, and this
line bas in circuit a telephone T and an interrupting apparatus U provided with a
separate battery V.
This interrupter is employed to cause distinct indications in the telephone of the
presence of current in the line S. In Figure 3 a galvanometer W is shown in the
line S, in place of a telephone and interrupter, to indicate the flow of current
through the line.
Instead of using the interrupter U, the portion of the wire F around the disc R',
with which the slide K comes in contact may be roughened so that whenever K, is
moved over any part of the wire it will produce a rapid interruption of the contact
or variation of current which will be readily heard in the telephone if there is
any current passing over the line S; or any suitable current varying device which
will cause a vibration of the telephone diaphragm may be used in place of the
interrupter. We have also found that by using a small magneto generator in place
of the main battery Q, the presence of a current in the line S will be indicated in
the telephone, by reason of the variation in the strength of the current from the
main circuit, without an interrupter. The connections of the battery and the
line S may be interchanged, that is, the battery may be connected at C and I, and
the line S connected at D and E and have the apparatus operate properly.
Where it is desired to determine temperatures below 400° Fah. copper and
German-silver will be found advantageous materials to use in the respective
temperature coils, because of the cheapness of those metals, their permanence, and
wide difference in temperature coefficients of electrical resistances.
German-silver is especially adapted for one of these coils, because of its low
electrical temperature coefficient and high specific resistance.
These differences in the temperature coefficients of electrical resistance between
these two metals are expressed according to the formula following:-
=

page_text: where 7, represents the resistance at 0° of temperature and r₁ the resistance
at to of temperature and for
Copper, a + 0.003824 and b = +000000126 and for
German silver, a = + 0.000443 and b = + 0.00000015.
Among the metals which may be used in place of copper are gold, aluminium or
iron for low temperatures and platinum for high temperatures, and alloys of various
metals, for instance what is known as platinoid may be used, in place of German-
Bilver Gold and aluminium have the commercial disadvantage of large expense
and iron the practical one of liability to corrode.
Platinum alloyed with iridium will be found advantageous for use instead of
German-silver when high temperatures are to be determined, or carbon may
be used.
Although the galvanometer ordinarily would serve only to indicate the presence
or absence of an electric current in the line S and thus aid in adjusting the pointer
of the ratio-measurer, it may be used however, to indicate absolute temperatures,
by first fixing the sliding contact at a given position and then by practical
trial properly graduating the scale of the galvanometer to indicate degrees of
temperature. Then any change in the ratio of resistance between the two
temperature coils, caused by a change in temperature at that point, will be
automatically indicated by the galvanometer needle on its scale at each moment.
Such an arrangement of a galvanometer would be of especial benefit in very
accurate temperature measurements. By using a registering galvanometer, of the
kind which throws a ray of light upon a slowly moving sensitized sheet, and
graduating this sheet to indicate degrees of temperature the variations of
temperature at the temperature cils through a period of time may be accurately
and automatically registered.
In the construction of our improved apparatus to be used out of doors, to
determine the temperature of water at various depths for instance, for the
temperature coils we have generally used cotton or silk covered wires placed
within small metal or glass tubes to serve as a protection; when using a small
brass tube. the whole may be placed in compact form by coiling into a helix,
and this resistance coil may be further protected against corrosion and pressure by
filling the spaces around the wire within the tube with oil, or other insulating
substance which may be easily introduced, thus furnishing a covering for the wire,
through which changes in temperature are readily and quickly transmitted.
We have found that a resistance of about 100 ohms for each coil is satisfactory
for use with a telephone, although we have had no difficulty in using coils having
a resistance of 1000 ohms or even more.
It will be found convenient to make the resistance of the two temperature coils
equal at a temperature about the middle of the range of degrees which the
apparatus is intended to be used for determining.
The resistance allowable in the two leading or line wires L, L', should depend
upon the resistance of the temperature coils, the variations of temperature to which
the leading wires are liable to be subjected, the range of temperature readings
which are to be made, and the precision desired.
When the range of temperature readings desired is 100° and when both leading
wires are of copper and liable to be subjected to variations of 90° Fah., the
resistance of each of those wires may be as great as two per centum of either
of the temperature coils and give accurate results, with one tenth of a degree
Fahrenheit as a maximum error; while for ordinary commercial work when the
range of temperature readings desired is 50° and the leading wires are of copper
and liable to be subjected to variations of temperature of 50°, the resistance of
each of these wires may be as great as 20 per centum of either of the temperature
coils and give accurate results with ° Fah. as the maximum error; but to obtain
this result the two leading wires must be kept at the same average temperature,
which may be readily accomplished by twisting them together, and for convenience

page_text: the wire leading to the ratio-measurer through the telephone or galvanometer may
also be twisted with the other leading wires. The amount of resistance of the
conductor leading to the telephone, with relation to the temperature coils is not
material however.
Referring to Figure 1, the operation of the apparatus will be readily understood.
From the battery Q, the current from the positive pole passes to the terminal D
where it divides, a portion passing over the conductor L, through the
resistances X, Y, in series, and back over the conductor L¹ to E and thence
to the negative pole of the battery; the other portion of the current passes
from D around the disc R' over the two portions A, B, of the conductor F, to the
terminal E, and back to the negative pole of the battery..
When the temperature at the location of the coils X, Y, is such as to make the
resistance in them equal, the adjustment of the sliding contact K upon the wire F,
to produce a balance, should be arranged so that the pointer P preferably will be
at about the middle part of the scale M. The apparatus illustrated in the
drawings is in principle of operation, substantially like that of Wheatstone's
bridge, the coils X, Y, forming two arms thereof, and the portions A, B, of the
wire F, forming the other arms. When the two sides of this bridge are in balance,
no current will flow from the battery over the said line S; as soon however as the
temperature at the coils X, Y, changes, the ratio of resistance in those coils becomes
different by reason of the respective metals of which they are composed having
different temperature coefficients of electrical resistance as heretofore explained.
This change in the ratio of resistance will cause a difference in the potential of the
electric current at the two ends of the line S, and cause a current to flow over that
line, which will be indicated in the telephone or by the galvanometer. The
potential between the two ends of the line S may then be equalized by moving the
sliding contact K in one direction or the other around the disc R¹ until the
telephone or galvanometer indicates that no current is passing over the said line.
The mark where the pointer P then rests will indicate the temperature at the
temperature coils X, Y, for the reason as above set forth that the scale M has been
graduated to indicate the temperatures at which the various ratios of resistances
are produced in the said temperature coils.
To express the relation of the ratios of resistances in the several parts of the
apparatus in algebraic terms, when the current indicating device indicates a neutral
X A
electrical condition :- = but as X and Y vary with the temperature and A
Y B
and B by the slide K,
✗
B
X
must be varied as varies with the temperature.
Y
A
But is a function of the absolute temperature at X, Y, and therefore is the
B
same function of the absolute temperature at the location X, Y. In our apparatus
A
the ratio
B can be directly read from the graduated card, but we have found it
better to make the scale divisions such a function of that the absolute
temperature of X, Y, is indicated thereon.
In the above algebraic expressions the resistances of the leading wires L, L',
have not been represented for the reason that if L, L', were subject to no changes
of temperature the amount of resistance in these wires would be wholly immaterial
so far as the accuracy in determining temperatures is concerned, and even the
changes in resistance due to changes in temperature in those wires which do take
place are slight, and as the changes occur on opposite sides of the bridge at the
same time, and to substantially the same amount, in our apparatus, such changes
in resistance cause little error in its indications.
We have found that with our instrument when using a commercial telephone
as the current indicating instrument, accurate readings may be obtained to
0.05° Fahrenheit.

page_text: Having now particularly described and ascertained the nature of our said inven-
tion, and in what manner the same is to be performed, we declare that what we-
claim is:
1. An apparatus for determining temperatures electrically, consisting of two
conducting materials having different temperature coefficients of electrical resistance
and which are to be placed at the point where temperature is to be determined, and
a device for measuring the ratios of resistances produced by temperature changes
in said materials, electrically connected therewith, and to be located at a place of
observation.
2. An apparatus for determining temperatures consisting of two materials which
have different temperature coefficients of electrical resistance, and a device for
measuring the ratios of resistances between said materials electrically connected
therewith, and an electrical generator to supply current to said apparatus for
the purpose described.
3. An apparatus for determining temperatures consisting of two electrical
conductors which are joined by one extremity and have different temperature.
coefficients of electrical resistance, a device for measuring resistance ratios between
said conductors, electrically connected therewith, a source of electrical supply
therefor and a current indicating device in an independent line electrically
connected with said conductors and the ratio-measurer substantially as described.
4. An apparatus for indicating temperatures at distant or inaccessible locations.
consisting of a pair of resistance coils joined at one of their extremities, and which
have different temperature coefficients of electrical resistance, a device for
measuring resistance ratios between said coils, electrically connected therewith, an
independent line electrically connecting the junction of said coils and said ratio
measuring device, which has a telephone and a current varying device in circuit,.
for the purpose specified.
5. In an apparatus for determining temperatures electrically a pair of resistances,
which are constant in mass, but whose ratios of electrical resistance are different
at each degree of temperature, and a device for measuring said ratios consisting of
an electric conductor connected with said resistances and a movable contact upon
said conductor which is electrically connected with the junction of said resistances,.
through a current indicating device substantially as described.
6. In an apparatus for determining temperatures at distant or inaccessible
locations, two materials which have different temperature coefficients of electrical
resistance, joined together in proximity to serve as electrical resistances at the
distant point, and a Wheatstone slide-wire bridge provided with a source of
electrical supply and a current indicating device electrically connected with said
materials,
7. In an apparatus for determining temperatures electrically a pair of electrical
conductors which have different temperature coefficients of electrical resistance,
and are joined together in proximity, a device for measuring ratios of resistance-
between said conductors, electrically connected therewith, a source of electrical
supply, an adjustable contact on said ratio-measurer independently connected with
said conductors, and a scale in juxtaposition to said adjustable contact to indicate
degrees of temperature corresponding to the different ratios of resistance which
each temperature will produce in said conductors.
Dated this Twenty-second day of March 1895.
GEORGE CHANDLER WHIPPLE,.
Newton, Mass., U.S.A.
HENRY ELLIS WARREN,
Newton, Mass., U.S.A."