121 
1897 - 98 .] Thermodynamics of Volta-contact Electricity. 
III. (Adiabatic.) Let the slab move further inwards very 
slowly till the temperature of the whole apparatus, ideally 
coated with impermeable varnish, sinks to t. 
IY. (Isothermal.) The whole apparatus being kept by proper 
surface appliances at constant temperature t, draw the 
slab outwards to its primitive position. Let H be the 
quantity of heat which must be removed to prevent 
lowering of temperature. 
§ 7. Eemark that if Q of § 5 is positive, H' and H are both positive 
and H'>H; but if Q is negative, H' and H are both negative and 
-H > - II'. In either case we have essentially by my definition 
of absolute temperature {Math, and Phys. Papers, vol. in. Art. 
XCII. part ii. §§ 34-35) 
H' _ if . 
H “ t ’ 
in the former case t' >t, in the latter t'<t. 
By working out analytically all the details of this cycle of opera- 
tions, and taking into account Joule’s law of equivalence between 
heat and work, we arrive at the result 
JQ = c?Y/c7(log t) 
given as our final result in equation (7) below. 
But we arrive at it more easily, and in some respects more con- 
veniently, by founding on the doctrine of motivity, as follows : — 
§ 8. Let Y be the Yolta-difference of potential in air or ether 
between the opposed X and Y metallic surfaces. 
Let £, be the electrostatic capacities of the variable condenser 
CC, II, for two positions of the movable slab CC, which for brevity 
we call position £ and position 
The work required to pull out the slab from position £ to posi- 
tion % will be 
ms-o- 
The quantity of electricity passing from II across J to KK during 
this operation will be 
and therefore the quantity of heat which would have to be removed 
