PLATINUM SURFACE AT HIGH TEMPERATURES, 
23 I 
0'010012 ohm. From similar observations it was 
deduced that the electromotive force of the Clark’s 
cell (No. 5217 A) had fallen by 0'2 per cent, during 
the last two years. The variations being in opposite 
directions tend to cancel, and the outstanding error 
as far as the energy measurements are concerned is 
not sufficient to affect the results. On the other hand 
the temperature measurements are, of course, inde¬ 
pendent of the absolute value of the standard. 
The Clark’s cell was kej^t in the inner chamber of 
a Berthelot calorimeter; only very slow variations of 
its temj^erature were therefore possible, and the agree¬ 
ment of the electromotive force with the temperature 
indicated by tbe attached thermometer was thus en¬ 
sured. 
The enclosure was formed by a steel cylinder 
6'0 centims. in outside diameter and 2’06 centims. bore 
and 45 centims. long (see fig. 1). It was surrounded 
by a water jacket. The temperature of the enclosure 
was estimated by a thermometer, the spherical bulb of 
which just fitted a hole drilled in the walls of the steel 
cylinder, half-way along its length ; the depth of the 
hole being such that the bulb was equidistant from 
the inside and outside face of the wall. After the 
thermometer was in position the hole was filled with 
mercury to ensure satisfactory contact. 
The ends of the steel enclosure were closed by the 
fibre plugs (P^, Pg, fig. 1), held in position by the gun- 
metal nuts, N|, Ng. The current was led in and out by 
the copper bars, B^, B^ (about 1 centim. in diameter), 
passing through the centre of the plugs. One end of 
the radiating wire II was silver-soldered to the copper 
terminal B^, the other end was connected to B^ by a 
flexible connection (200, No. 40, S.W.G.). The radia¬ 
ting wire was kept taut by means of the light 
spring S. One of the jDotential terminals T^ was con¬ 
nected to the steel cylinder, the other kept insulated 
and passed through the fibre plug P^. The enclosure 
was kept at a constant temperature by means of a 
water circulation. 
The same pure platinum wire was used as a radiator 
'Q 
l|l 
Scale = alDOut 4 
