Polarization of Platinum Plates. 493 



was no deflection on the reflecting-galvanometer. The E .M.F. 

 of B 2 is then equal to the difference of potential between the 

 points P, Q in the primary circuit. 



Fie-. 4. 



The tangent-galvanometer had two equal parallel coils of 

 approximately square section placed at a distance apart nearly 

 equal to their mean radius, which was about 18*25 centim. 

 Each coil consisted of 22 layers of 20 turns each. The de- 

 flections were read by a telescope placed underneath the scale, 

 which was straight and was divided into centimetres and 

 millimetres. The distance from mirror to scale was 345*44 

 centim. The galvanometer-constant y was calculated to be 

 1/0*004618, so that, neglecting torsion, the absolute strength of 

 a current measured on this galvanometer is 0*004618 H tan 6. 



Torsion. — On turning the torsion-head through 360° the 

 needle was deflected through 118'. The factor needed to 



correct for this effect is therefore — 7-——-== 1*0055. 



21482 



Value of H. — The value of H for the position of the galva- 

 nometer was ascertained by comparing the time of vibration 

 of the magnetometer-needle in that position with the time of 

 vibration in the place for which the value of H had been 

 accurately ascertained. The mean of two determinations of 

 the period of vibration was 4*314 seconds. In the room 

 where the value of H was '1830, the time of vibration was 

 4*2085 seconds ; so that the value of H in this spot was 

 -183 x (4*2085) 2 ^-(4*314) 2 , whence H=**1742. 



Readings. — The resistance in the main circuit was about 

 4000 ohms, and 10 microfarads was the capacit} r employed, 

 and the arrangement was sensitive to within '25 per cent, of 

 the quantity to be measured. Readings of the galvanometer 

 were taken on both sides of zero, and the mean of four, which 

 were very close together, gave 



