128 BarreET1— On some novel Thermo-flectric Phenomena. 
temperature of the junction rose from a black up to a bright white heat; the cool 
junctions being throughout kept in water at the temperature of the room, or in ice. 
Here, then, we have the thermo-electric force arrested when a certain temperature is 
reached, and remaining nearly the same in spite of the increasing difference of 
temperature between the cool and hot junctions. We know that so long as the 
difference of temperature between the cool and hot junctions in any thermo-couple 
remains constant, and the circuit is unchanged, the potential difference also remains 
constant; but if the difference of temperature between the cool and hot junctions 
alters, then the potential difference, as arule, also changes; and for small changes of 
temperature the electro-motive force thus set up is, in most cases, proportional to 
the change of temperature. This, of course, is the principle upon which the 
thermo-electric pile is used as a delicate thermometer ; and with certain alloys this 
proportionality holds good through a wide range of temperature. 
In order to measure the exact temperature of the hot junction, a thermo-electric 
couple, formed of a platinum wire twisted with a wire of an alloy of platinum 
containing 10 per cent. of rhodium, was employed. With this couple the E. M. F. 
steadily rises, as the difference of temperature between the hot and cold junctions 
increases, through an enormous range of temperature. ‘This pyrometer, devised by 
M. Chatelier, has the great advantage of occupying a very small space, and very 
rapidly assuming the temperature to which it is exposed. For pyrometric purposes, 
it is, of course, necessary first to plot a curve expressing the relation between 
temperature and the resulting EK. M. F. This curve in the case of the Chatelier 
couple is approximately a parabola; but as Professor Callendar, F.R.S., has shown, 
in the course of his admirable researches on the platinum resistance pyrometer, the 
departure from a true parabolic curve is considerable when a wide range of 
temperature is to be measured by a Pt and Pt-Rh thermo-couple.* The vapour of 
boiling water (100° C.) and of boiling sulphur (445° C.) are the most convenient 
and reliable fixed points for plotting the lower part of the scale.} For the higher 
parts I have used the freezing point of pure silver (961°C.), and of potassium 
sulphate (1066° C.).£ 
A reflecting galvanometer was employed, a dead beat high-resistance instru- 
ment, of the D’Arsonval type, made by Ducretet, of Paris. Owing to its high 
resistance the alterations in the resistance of the circuit during the heating and 
cooling of the couple introduced no sensible error, the deflections being propor- 
tional to the E. M. F. and not to the current. 
* Philosophical Magazine, February, 1899. 
+ The thermo-couple must, of course, be protected from the sulphur by being enclosed in a hard-glass 
tube. To obtain the boiling-point of sulphur it is best to employ a hard-glass flask with a long neck, all 
except the lower portion of the flask being jacketed with asbestos. A convenient arrangement is supplied 
by the Cambridge Scientific Instrument Company, this I used. 
t Heycock and Neville, Zrans. Chem. Society, 1895, 
