Ratio of the Electromagnetic to the Electrostatic Unit, 323 



so. I therefore conclude that the difference between the ob- 

 served capacities of condenser and charging system together 

 and of charging system alone is a true measure of the capacity 

 of the condenser. 



4. Displacement of the Ball. — The upper half of the 

 spherical shell was lifted and the lower half adjusted upon 

 its supports until the distance of the ball from the shell was 

 the same at all points on the equatorial circumference. The 

 upper half of the shell was then replaced, and by means of the 

 rack and pinion the ball was first lowered and then raised 

 until it touched the shell, the exact moment of touching being 

 indicated by an electrical contact, and several readings taken 

 on the vernier in each position. The mean of the readings 

 in the two positions gave the central position. In this 

 manner the ball was adjusted vertically to within 01 mm., and 

 equatorially within 0'2 mm. Thus the ball is centred to 

 within less than one per cent, of the distance between the ball 

 and shell, which is 25 mm. Thomson has investigated a 

 formula for the capacity of eccentric cylinders. The formula 

 shows that for a displacement of one per cent, the capacity is 

 increased 2 lu °f one P er cen ^- Evidently the capacity of 

 spherical shells is less affected by slight eccentricity than that 

 of cylinders. Therefore we may safely conclude that no error 

 is due to eccentricity. This conclusion was verified experi- 

 mentally, a displacement of four per cent, causing an in- 

 appreciable change. 



5. Adjusting Resistances. — The accidental errors occurring 

 in adjusting the resistance a so as to produce zero deflexion 

 will be eliminated by a large number of observations. Their 

 magnitude depends on the strength of the current, delicacy of 

 the galvanometer, regularity of the vibrator, &c, and is 

 larger with the slow fork than with the fast. The stronger the 

 current and the more sensitive the galvanometer, the greater 

 the deflexion due to a certain error in the resistance a ; but, 

 on the other hand, the greater the unsteadiness of the spot, 

 so there is a practical limit in that direction. That these 

 accidental errors are small is, I think, attested by the uni- 

 formity of the results obtained. 



Electrostatic Capacity. 



The electrostatic capacity of the condenser was calculated 

 from the formula 



