331 
CONDUCTIVITY OF SELENIUM CRYSTAL 
where T^, T 2 and T 3 , are respectively the temperatures of the 
top, middle, and bottom copper discs. C is a constant. So this 
apparatus will not give the absolute thermal conductivity of the 
selenium, but will give the ratio of that conductivity to that of 
glass. If we assume, as we are surely justified, that light does 
not affect the thermal conductivity of glass, then we are in a 
position with this apparatus to discover if there is any change 
in the thermal conductivity of the selenium with illumination. 
The illumination was obtained by means of a 25 watt tungsten 
lamp, having a full frosted globe, placed at L in figure 27. 
The connections at the right hand side of the diagram are 
perhaps clearly enough indicated. By the proper manipula- 
tion of the keys it is possible to get the temperature differences 
(when galvanometer readings are reduced to temperatures) be- 
tween the various copper discs. Also it is possible, by throwing 
the key K downward, to place a cell in circuit, and thus get 
the measure of the electrical conductivity by means of the re- 
sulting galvanometer deflection. 
Without going more into details in regard to the experiment, 
the table below is referred to for a brief indication of the re- 
sults. In this table the first six columns represent respectively 
the temperature difference between the top and middle discs, 
between the bottom and middle discs, between the room tempera- 
ture and that of the middle disc, the temperature of the top, 
of the middle, and of the bottom discs. The seventh column 
represents the average temperature of the selenium crystal; 
eighth column the relative electrical conductivity of the crystal ; 
the ninth column represents the ratio of the thermal conduc- 
tivity of the selenium to that of the glass; the tenth column 
represents the ratio of the electrical conductivity in the light 
to that in the dark : and the last column represents the state of 
the selenium. It will be noticed that the effect sought for is 
missing, or at least is so small that it becomes inappreciable. 
There seems a general tendency for the thermal conductivity 
to increase With increased temperature of the selenium, but 
there does not seem to be any tendency for any change with the 
illumination. This is to be noted in connection with the fact 
that the same illumination increases the electrical conductivity 
nearly three times. 
