JANUARY 15, 1914] 
NATURE 
569 
MODERN METHODS OF MEASURING 
' TEMPERATURES.1 
HERE are few manufacturing processes in which 
the question of temperature is not involved, and 
it may be of use to review briefly the methods now 
available for the measurement of temperatures. 
As a result of the work of Guillaume,* Chree,* the 
Physikalische Technische Reichsanstalt, and others, 
the mercury thermometer has become an instrument 
of considerable precision. Hard glasses such as the 
French verre dur and the Jena glasses 1641 and 594i, 
are now used almost universally for making at least 
the bulbs of the best thermometers. In all mercury 
thermometers intended for accurate work the two 
fundamental points (0° and 100° C.) are introduced 
whatever may be the range of the thermometer ; this 
is done by making small enlargements in the capillary. 
Assuming that such a thermometer has _ been 
properly constructed and its corrections determined at 
one of the National Physical Laboratories, it is pos- 
sible to measure temperatures with it to an accuracy 
of ooo1° C., throughout its range. 
For nearly all engineering work an accuracy of 
ooor® C, is not required,‘ and the recent develop- 
ments in mercury thermometers have been in the 
direction of making them easier to read* and more 
robust. The introduction of an inert gas under pres- 
sure above the surface of the mercury in the tube of 
the thermometer raises the temperature at which the 
mercury boils, thus permitting the thermometer to be 
used up to a temperature of 540° C., this limit of 
temperature being due to the softening of the glass 
envelope. 
Thermometers of various kinds have been developed 
in which metal tubes have been substituted for the 
glass envelope, and gases, saturated vapours, or 
liquids for the mercury. Mercury-in-steel instruments 
are proving themselves very practical instruments in 
engineering work. They consist of a steel bulb to 
which a steel capillary tube is attached, the latter 
being connected to a form of Bourdon pressure-gauge. 
The whole system is filled with mercury and hermetic- 
ally sealed. The hand, attached through some simple 
mechanism to the pressure gauge, is arranged to 
point over a dial or to carry a pen which writes on a 
circular sheet of paper rotated by clockwork. The 
recording or indicating mechanism may be placed at 
distances up to 75 ft. from the bulb of the thermo- 
meter. 
| 
Thermo-electric Thermometers. 
Expansion thermometers have a limited range of 
temperature over which they may be employed, and 
some of the other physical properties of materials 
must be used for the determination of high tempera- 
tures. The most valuable for this purpose is the 
property by which a thermo-electric force is set up 
when a junction of two dissimilar metals is heated 
when this heated junction forms part of a closed 
circuit, the magnitude of the current and its direction 
depending on the metals employed. Le Chatelier 
showed that platinum, platinum-rhodium (10 per cent. 
Rh) was the most satisfactory of all thermo-elements, 
and this has been generally adopted as the standard 
1 Abridged from a paper read before the Institution of Mechanical 
Engineers by Robert S. Whipple. 
2 Traité Pratique de la ! hermométrie de Précision, C. E. Guillaume. 
3 Philosophical Magazine, March and April, 1293, C. Chree. 
4 As an mstance when enginerr~ have been keenly in'erested in tempera- 
ture measurement to this degree of accuracy, mention must he made of the 
work of Prof. Barnes, on frazil ice. He has shown that differences in the 
water temp:rature of o’oo1° U. may bring about the formation of frazil ice 
which may throw out of action a complete hydro-electric plant. In his case 
all the measurements were made with resi-tance thermometers. See ‘Ice 
Formation, chap. v., H. T. Barnes. (London: Chapman and Hall, Ltd.) 
5 £.g. the lens-front thermometer invented by Luigi Peroni. 
NO. 2307, VOL. 92] 
couple. Owing to the high price of platinum many 
attempts have been made to find satisfactory thermo- 
couples made of comparatively inexpensive wires. 
The most satisfactory of these so-called “base” 
metal couples is silver-constantan (the latter being 
an alloy wire sold commercially as a resistance mate- 
rial, and consisting of copper 60 per cent., nickel 
4o per cent.), and it may be employed up to 700° C. 
Copper is frequently used instead of silver as one 
element of this couple, but in practice it will not be 
found so trustworthy as silver for temperatures above 
500° C 
For temperatures from 700° C. to 1100° C, the 
Hoskin’s couple, which consists of nickel used in 
conjunction with nickel-chromium (ro per cent. Cr) 
may be employed. 
The electromotive forces given by various thermo- 
couples differ very, much, as the following table will 
show. In each case the cold junction temperature 
is o° C, 
Approximate electro- 
motive force in milli- 
volts at 500° C, 
Io per cent. 
Name of thermo-couple 
Platinum-platinum, 
rhodium ae os: sine 44 
Platinum-platinum, 10 per cent. 
iridium 33 sete aor ye 74 
Nickel-nickel, 10 per cent. chro- 
mium (the Hoskin’s couple) ... 10-0 
Tron-nickel ... ae ae ae. 
Jron-constantan 26-7 
Silver-constantan 27-6 
Copper-constantan 278 
The relation between temperature and the E.M.F. 
produced by a thermo-couple when the cold junction 
is maintained at 0° C. is given by Holman’s empirical 
formula ° :— 
log E=A log t+ B, 
when E=E.M.F. of the thermo-couple in microvolts ; 
t=the temperature of the thermo-couple in degrees 
Centigrade, and A and B are constants depending 
on the wires employed. For the chief thermo-couples 
in general use at the present time this equation is 
as follows :— 
Platinum, platinum-rhodium, approximately log E= 
1-19 log t+0-52. 
Platinum, platinum-iridium, approximately log E= 
1-10 log t+0-89. 
Silver-constantan, 
t+ 1-34. 
In accurate thermo-electric work the universal prac. 
tice is to immerse the cold junction of the thermo- 
couple in melting ice and to adopt the potentiometric 
method of measuring the electromotive force given. 
In industrial practice the E.M.F. is measured directly 
by a galvanometer, which should be placed in a spot 
Which is not subject to great variations in tempera- 
ture. It is only necessary, therefore, to run the wires 
from the hot end of the thermo-couple straight to the 
galvanometer, and this is the course generally fol- 
lowed in the case of the base metal couples. Owing 
to the costliness of the material, it is impossible to 
do this with platinum couples, and several proposals 
have been made to overcome this difficulty. The most 
satisfactory method is one originally due to Bristol,’ 
but suggested independently by Peake,’ in which 
an inexpensive alloy is substituted for the platinum 
wires, the alloy being so chosen as to give the same 
E.M.F. against copper as that given by the platinum, 
platinum-alloy couple. The resultant E.M.F. gene- 
rated by this compound couple is the same as if the 
approximately log E=1-14log 
6 Phil. Mag., x\i., p. 465, 1296. 
7 British Patent Specification, No. 14544. A.D, 1904. 
8 [bid., No. 370,A.D. 1909. > 
