Plotting Curves by the Aid of Photography. 



A simplification of this method by which 

 the need for the separate time-curve is done \ 

 away -with has been worked out success- z 

 fully and has given us good results, in the \ 

 Montefiore Institute. The photographic 

 record is produced by a spark obtained 

 from a Ruhmkorff coil of ordinary dimen- 

 sions. The induction-spark passes between 

 a strip of magnesium and a carbon-point, 

 like that which is used in an arc-lamp. 

 The two electrodes are fixed less than I 

 millim. apart. The spark is thrown on a 

 concave mirror attached to a galvanometer- 

 needle, or other body whose motions are 

 to be observed, and reflected thence to the 

 sensitized paper, covering either a rotating 

 cylinder, or a frame descending between 

 guides. The periodicity of the spark de- r 

 termined by the elasticity of a spring used i 

 in the inductive break- make, shows natu- '- 

 rally the time-interval by divisions on the 

 curve corresponding to equal accurately 

 Itnown intervals of tune. The breaks are 

 effected by an electric tuning-fork of known 

 period. A single circuit is then formed, 

 including the battery, the primary wire, 

 and the coil of the tuning-fork. 



A short white spark is obtained, the 

 position of which is invariable, by connecting 

 the terminals of the secondary coil to the 

 coatings of a Leyden jar. Besides, it is 

 well to reduce the image of the spark by 

 introducing a double concave lens in the 

 path of the reflected ray. 



By way of illustration of the method we f 

 give a curve representing the variation of 

 magnetic field in the air-space of an electro- z 



motor. A short length of straight insu- \ 



lated wire was fixed to the armature pa- 

 rallel to the axis. This wire was connected 

 at its end to rings, insulated and fixed 

 on the axle of the motor. Brushes press- 

 ing on the rings were connected to a 

 galvanometer, of which a description is 

 given above. The ordinates of the curve 



181 



