ELECTRIC DISCHARGE WITH THE CHLORIDE OF SILVER BATTERY. 
87 
experiment 17°*5 C., and if the gas had been allowed to expand, its volume would 
3787x 71-8 
have been 
56 
= 4855 cub. centims., or an increase of 1068 cub. centims. ; as the 
gas was kept at a constant volume the pressure increased in the ratio 1*282 to 1. 
Supposing the expansion to be due to an increase of temperature, it would follow that as 
the volume was kept constant the pressure would vary as the absolute temperature,' 5 ' 
therefore ^- = ^vr= 1'282, whence T'= 1'282 X 291*2=373°*3 C. The temperature 
lob 
while the discharge was passing would be (373'3 —273*7) = 99 0, 6 C., consequently the 
rise of temperature (99-6 — 17*5) = 82°*1. The temperature of the bell-jar as determined 
by a thermometer enclosed in it with its bulb uppermost only rose 0°'64 C. per second, 
taking into account the rate of cooling. It is evident, therefore, that the increase of 
pressure cannot be ascribed to the instantaneous heating of the bell-jar 82° C. 
A photograph of the discharge obtained in 60 seconds, and copied in Plate 9, 
tig. 8, shows that the central spindle or arc-proper could not have been so much 
as half-an-inch, 1*27 centims., in diameter, and its length we know was 6 inches, 
15‘24 centims. ; the cubical content of a cylinder of these dimensions is 19'3 cub. 
centims. If we assume that this volume increased 1068 cub. centims., or to 19*3 + 
1068= 1087*3, then ^j^= 56*337 represents the number of times it increased, and 
accordingly T' = 56*337 T=56*337 X 291*2 = 16,405° C., and 16,405 —291*2 = 16,114° C. 
would therefore be the temperature of the arc. 
Experiments were made to ascertain roughly the temperature of different parts of the 
arc, and for this purpose wires were supported by one end being twisted round a vertical 
glass rod, the other end being made to project into the arc at different heights, or else 
wires were strained through opposite holes A, h', A", &c., drilled at different heights in 
the side of a glass tube 1*75 inch diameter, which was placed inside the bell-jar, as 
shown in fig. 81. Platinum wires 0’0125 inch diameter were not fused in any part of 
the arc ; they were heated to a white heat in the luminous but not in the non-luminous 
part. But platinum wires O'OO L inch in diameter, supported by wire 0'0125 inch, in 
various parts of the arc were immediately fused ; the temperature of the arc was 
therefore as high as the fusion-point of platinum, and possibly considerably higher 
(Plate 9, figs. 9, 10). 
The whole of the heat evolved by a current of 0'01102 W through a resistance of 
600,000 ohms would raise 73T grammes of air 1 = C. per second ; and if communicated 
to the ah- in the bell-jar, weighing 0'339 gramme, would have raised its temperature to 
73'1 
==v=215°*6 C. in that time. We know from direct experiment that such an enormous 
U Oou 
evolution of heat was not communicated to the air in the bell-jar, because its tempera¬ 
ture only increased about 0°*64 C. per second; and it would have to be assumed that 
the rest of the heat escaped almost instantaneously by radiation. It is difficult con- 
* Absolute zero=273v C., 273*7-|-17*5=291'2. 
