Photography to Electrical Measurements. 375 
The method we have used enables one to study the action 
of a cell at one’s leisure, the apparatus running at night or dur- 
ing the day when one is occupied with other work. eam 
so that the same beam of light may be reflected by both this 
murror and the one attached to the galvanometer needle. The 
through the galvanometer its changes in strength for different 
times are indicated by the relations of the two lines drawn 
upon the sensitive paper. The line drawn by the light from 
the Stationary mirror isa straight one, and serves for the abscissa 
i. 2. 
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of times, while the perpendicular distances from the curve drawn 
y the mirror attached to the needle of the galvanometer to 
this axis of times give the ordinates of the curve drawn bj 
the latter. Rapid printing paper was used and an ordinary 
gas flame gave a sufficiently strong spot of light to produce an 
actinic effect, 
