OP ARTS AND SCIENCES. 



231 



sought to calibrate the dynamometer with direct currents, and, by a 

 mathematical consideration of the curve obtained, to construct a curve 

 proper for alternating currents. 



In calibrating this dynamometer we met with considerable difficulty, 

 for it was far too sensitive to be put in direct circuit with the most 

 sensitive standard galvanometer, and it could not be placed in a shunt 

 circuit because of the impossibility of determining its equivalent 

 resistance, the inverse electro-motive force of the sulphuric contact ; 

 entering in as a doubtful element. t 



These difficulties were overcome by setting up the dynamometer 

 with the planes of its outer coils east and west, connecting it in series 

 with a very sensitive bell-magnet galvanometer, and observing the 

 corresponding deflections. Series were taken by varying the resist- 

 ance in circuit, and also reversing the current both at the battery and 

 in the inner coil. The bell-magnet galvanometer having been cali- 

 brated by a standard instrument, we computed the currents * cor- 

 responding to the different deflections, and plotted the curves of 

 deflection and current for the dynamometer. These curves were 

 found to be two equal parabolas, whose equations were 



and 



<7 2 - 1.297 C = -0.134 5, 

 C 2 - 1.297 C = 0.134 5, 



(1) 

 (2) 



C being the current in milliamperes and S the scale readings in 

 millimeters. (Figure 1.) 



Fig. 1. 



* A variation of 0.00001 ampere in the current could be measured. 



