Barrett—On some novel Thermo-Electric Phenomena. 129 
The value of the scale-reading of the galvanometer in 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 the E. M. F. in microvolts at three points ought to be 
as follows :— 
LOOZC! : : ; 650 microvolts 
445° : : > BOBO 
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 Callendayr’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 Plate [Xa., 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 were 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 
junctions 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 
temperature were obtained ; the readings which corresponded to similar tem- 
peratures in heating and cooling being remarkably concordant, allowance being 
made for a very small constant difference to be mentioned in the sequel. 
The results are plotted in the second curve shown on Plate IXa. It 
will be seen that, up to a temperature of 320°C., the E. M. F. of the nickel-steel 
alloy and iron couple rose rapidly; it then remained absolutely constant until 
* 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 microyolts at 100° and 445°C. is 
found, to determine the higher points in the curve by means of the ratio given aboye: the scale-readings 
being plotted as ordinates and the temperatures as abscisse. 
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