Page:Collected Physical Papers.djvu/250

230 would continuously increase the E. M. F. acting on the circuit containing the sensitive substance (fig. 51). If the galvanometer spot of light is thrown down on the platform by a mirror, suitably inclined, and if

the movement of the spot of light, due to variation of current, takes place at right angles to the motion of platform, then a continuous curve would be traced on the platform, of which the abscissa would represent the E. M. F. and the ordinates the currents. This curve could be fixed if we laid a sheet of photographic paper on the platform, or we might obtain it by the simpler expedient of following the track of the galvanometer spot with a pencil.

I used squared paper for recording the curves. It is divided into inches, and these further sub-divided into tenths. The potentiometer wire is 20in. long, and the length of the platform is the same. As the platform