128 Barrett — On some novel Thermo- Electric Phenomena. 



temperature of the junction rose from a black up to a bright white heat ; the cool 

 junctions being throughout kejit in water at the temperature of the room, or in ice. 

 Here, then, we have the thermo-electric force arrested wlien 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 a rule, 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 princiiDle 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 jilatinum 

 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 E. 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 (4-45° C.) are the most convenient 

 and reliable fixed points for plotting the lower part of the scale.f For the higher 

 parts I have used the freezing point of pure silver (961° C), and of potassium 

 sulphate (1066° C.).J 



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. 



f 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. 



J Heycock and Neville, Trans. Chem. Society, 1895. 



