Novel Thermo-Electric Phenomena. 311 



microvolts was accurately determined by means of a standard 

 cell and potentiometer, one scale-division being found to be 

 equal to 26*5 microvolts. With a similar couple Professor 

 Callendar found that the E.M.F. in microvolts at three points 

 ought to be as follows : — 



100° C. . . 650 microvolts 



44:5° . . 3630 „ 



1000° . . 9550 „ 



the other junctions being in ice. My own determinations 

 at 100° C. and 445° C. gave almost identically the same values 

 in microvolts ; this may be accidental, but the variations of 

 thermo-electric power in different specimens of platinum is 

 probably very slight. I therefore took Professor Callendar's 

 value in microvolts for 1000° C. in preference to the rather 

 different number I obtained*. The results are plotted in the 

 curve shown in the figure, marked " pyrometer couple." 



We will now return to the thermo-electric behaviour of this 

 nickel-manganese-iron alloy. An iron wire, drawn from the 

 purest commercial iron, was coupled with a wire of the alloy. 

 The wires were twisted together at the junction and then 

 brazed. After insulation with asbestos they were lashed 

 alongside of the platinum-rhodium pyrometer couple, and the 

 pair of couples was then inserted in the centre of a thick iron 

 tube held horizontally in, and heated by, a gas furnace, the 

 temperature of which could be raised to a white heat. The 

 cooler j mictions of both couples were kept in ice, and pairs of 

 readings were taken as the temperature was raised, and again 

 as it was lowered ; the rise and fall being slow and steady in 

 both cases. As the readings of each couple were taken alter- 

 nately, to obtain a true comparison three readings were 

 necessary in each case : first, of one couple A ; then, of the 

 other B ; then, of A again ; the mean of the first and last 

 readings of A being comparable with B. The galvanometer 

 being extremely dead-beat, all three readings could be taken 

 closely together. This comparison was repeated two or three 

 times every minute during heating and cooling, thus several 

 series of readings for about every 10° C. rise or fall of tem- 

 perature were obtained ; the readings which corresponded to 

 similar temperatures in heating and cooling being remarkably 

 concordant, allowance being made for a very small constant 

 difference to be mentioned in the sequel. 



* As (within the limits of the scale) the E.M.F. of the thermo-couple 

 is directly proportional to the scale-readings of the galvanometer, it is 

 easy, when the E.M.F. in microvolts at 100° and 445° C. is found, to 

 determine the higher points in the curve by means of the ratio given 

 above : the scale-readings being plotted as ordinates and the temperatures 



Y2 



