Page:Collected Physical Papers.djvu/328

308 the abscissa the time. The curves are obtained directly, by tracing the excursion of the galvanometer spot of light on a revolving drum. The drum, on which is wrapped the paper for receiving the record, is driven by clockwork (fig. 84). Different speeds of revolution can be given to it by adjustment of the clock-governor, or by changing the size of the driving-wheel. The galvanometer spot is thrown down on the drum by an inclined mirror. A stylographic pen attached to a carrier rests on the writing surface. The carrier slides over a rod parallel to the drum. On stimulation, the resulting excursion of the spot of light is followed by moving the carrier which holds the pen; after, the cessation of stimulus the excitatory effect slowly disappears, and the galvanometer spot returns gradually to its original position. The response and recovery are thus directly traced on the recording surface. We can calibrate the value of the deflection by applying a known electromotive force and noting the deflection which results. The speed of the clock is previously adjusted so that the recording surface moves exactly through, say, one inch a minute; this speed can, however, be increased to suit any particular experiment. Very accurate records can thus be obtained in a very simple manner. A large number of records might be taken by this means in a comparatively short time.

Photographic Recorder.—The records may also be made photographically. A clockwork arrangement moves a photographic plate at a known uniform rate, and a curve is traced on the plate by the moving galvanometer spot of light. The records given in this paper are accurate reproductions of those obtained by one of these two methods.