30 Mr. A. H. Pfund: A Study 
answered just as well, this device was chosen on account of 
its simplicity and ease of manipulation. 
Fig, 
y} 
to) —_e 
O 
OBSERVER 
Be) 
Speco ea ee aS 
=== 




The galvanometer, G;, connected to the thermopile was of 
the fot: coil Thomson type, being of low resistance and high 
sensibility ; the other oalvanometer, Gs, connected to the 
selenium cell was of the same type, but had a higher resist- 
ance and lower sensibility, Both were effectively shielded 
against magnetic disturbances by three concentric cylinders 
of soft iron. 
On account of the large number of readings which must be 
taken in a work of this character, the changes in resistance 
of a cell were determined by using a method of direct de- 
flexion rather than by using the Wheatstone’s bridge. 
Although the data recorded are percentage-change in con- 
ductivity, the absolute value of this change in ohms may be 
determined readily as the initial dark resistance of a cell is 
always given. 
The specific nature of a cell determined the method ot 
procedure which was to be used in the measurements of 
resistance, for the difference between various cells was quite 

