599 
films formed by evaporation. We have found that these films have 
the additional advantage of the time-change of the resistance mentioned 
above being much smaller than with films formed by chemical depo- 
sition or cathode discharge, which circumstance is naturally very 
favourable to obtaining constant results. 
Ul. The experiments. 
The experiments were made with thin films of platinum, tungsten 
and silver. The platinum was Herauvs’s chemically pure platinum 
(@o—100 = 0.003905); that both the other metals were also very pure 
was shown for tungsten by the value of the temperature-coefficient 
of the electric resistance and for silver by chemical analysis. 
The formation of the films. The films were formed 
on the inner wall of a cylindrical glass tube (see fig. 1) 
tm) about 36 mm. wide. Inside about half way up the tube 
two silvered rings Rk, and &, were produced on the 
wall of the glass, at a distance of about 16 mm. from 
each other. From each of the silvered rings two platinum 
wires sealed through the glass lead to the outside. By 
‘measuring the resistance between the two wires connected 
to the same ring it was possible to make sure, that a 
Zz. good contact was established between the platinum and 
the silver deposit. At the top of the glass cylinder two 
nickel wires are sealed into which is fused the wire 
D of the metal to be tested. The wire has the shape of 
a long hair-pin, with its two legs only a few millimetres 
apart. The diameter of the wire was taken such, that 
only small parts at the ends of the wire did not glow 
by the cooling effect of the wires which admit the current. 
The part of the wire which is opposite the rings and 
| up. to a few centimetres above the higher ring showed 
Fig. |. a perfectly uniform glow. To the lower end of the glass 
cylinder a lamp was sealed in which a tungsten spiral was enclosed. 
The whole was thoroughly evacuated, while at the same time the 
glass wall was heated and the wire /) was made to glow, in order 
to free both from gases as completely as possible. 
After the tube was sealed off from the pump, the tungsten spiral 
was kept at a very high incandescence for some time, by which 
process the last remnants of gas were removed and a very high 
vacuum was obtained. The wire D of the metal under examination 
was then made to glow at a constant temperature regulated so as 
129, 
