126 THE ROYAL SOCIETY OF CANADA 



R is the contact of metal or alloy. It consists of two wires placed 

 at right angles to each other. 



^| — 



Fig. 1. 



X is a reversing key by means of which the current can be sent 

 through the contact wires in either direction. 

 K is a key in the main circuit. 



(b) Calibration of the galvanometer scale. 



A simple method was used in obtaining the calibration curve 

 of the galvanometer. First, the control magnet of the galvanometer 

 was placed so that, when all the current in the circuit passed through the 

 galvanometer, the deflected spot of light was at one end of the scale. 

 Then the contact wires were pressed together and the resistance in 

 the megohm was adjusted until the deflected spot of light was near 

 the other end of the scale. This condition was obtained when the 

 200,000 ohm coil was placed in the circuit, thus giving a current of 

 lxlO- 5 amperes. Next, a standard resistance box was substituted 

 for the contact wires; and by putting into the circuit various coils of 

 the resistance box it was possible to calibrate the scale for all de- 

 flections corresponding to resistances from infinity to zero. Fig. 2 

 shows the curve as plotted from a scale six times as large. 



The megohm prevented a sudden lowering of the E.M.F. of the 

 battery, so that there was no perceptible change in the current even 

 after the apparatus had been in use for several days. 



