600 
to make the evaporation take place at a suitable rate. By the 
evaporation the wire becomes continually thinner; in order to keep 
the temperature always the same, it is necessary that P< should 
remain constant’), where V represents the potential-difference between 
the ends of the wire and 7 the current in the wire. 
By a suitable choice of the voltage (storage cells) and resistance 
it is possible to arrange, that this condition is satisfied without having 
to change the voltage or the resistance, at least as long as the 
thickness of the wire does not diminish by more than a few 
percents, as was the case in our experiments. 
Determination of the thickness of the films. 
The thickness of the films was calculated from the quantity of 
metal deposited per square centimetre of the glass wall, taking for 
the specific gravity the value which holds for the metal in thick 
layers. As the wire glows at a constant temperature, the vaporized 
quantity is proportional to the time and to the surface of the wire; 
as only a small fraction of the wire evaporates, the diminution ot 
the surface of the wire was but small; the diminution was moreover 
taken into account. For tungsten the quantity which evaporates per 
second and per cm’. of the wire surface is known as a function 
of the temperature *). The constant temperature to which the tungsten 
was beated was calculated from current-strength and diameter of the 
wire; between the three quantities: current, diameter and temperature 
a certain relation holds which could be derived from the formula 
for the total radiation of tungsten as given by LANGMUIR*®) and from 
measurements made in this laboratory on the specific resistance of 
tungsten at high temperatures. 
For platinum too the velocity of evaporation has been measured 
by LANGMUIR*) as a function of the temperature; the temperature 
of the platinum wire was calculated from the total radiation using 
the formula given by LuMMER*). 
For the silver we determined the total quantity evaporated at the 
close of the experiments from the loss of weight of the wire; the 
total time being known during which the wire has been heated, the 
quantity vaporized per unit of time may be calculated. 
As a check on the calculations the total loss of weight of the 
1) Comp. I. LANemurr, Phys. Rev. Vol. IL, p. 335, ‘913. 
2) I. LANGMUIR, Phys. Rev. Vol. II. p. 329, 1913. 
3) I. LANGMUIR, Phys. Rev. Vol. XXXIV. p. 417, 1912. 
4) I. LANGMUIR, Phys. Rev. Vol.-IV, p. 377, 1914. 
5) O. LumMer: Verflüssigung der Kohle und Herstellung der Sonnentemperatur. 
p. 42. Vieweg 1914. 
