Electromagnetic to the Electrostatic Unit. 301 



shown. b, V (fig. 3), are two fine brass wires, uniting at n, and 

 tipped with platinum at p, p', where they are bent at right 





%S®lp 



■p' ' ' "■" ~n p / 



angles and fastened to the fork with an insulating cement. 

 Y, Y' are two small blocks of vulcanite attached to a firm sup- 

 port A. Below the platinum points are two cavities in the 

 vulcanite which are filled with mercury, and as the fork 

 vibrates, first one and then the other of the points dips into the 

 mercury. Thus the mercury cups, which are joined to B and 

 D, respectively (fig. 1), answer to the posts S and R, while the 

 wires b, V unite and run to the ball of the condenser. When 



the prongs separate p' dips into the lower cup and the con- 

 denser is charged ; when they approach p dips into the other 

 cup and the condenser is discharged. The points must be at 

 least half a millimeter above the surfaces of the mercury when 

 the fork is at rest, in order to avoid both dipping at once and 

 short circuiting the condenser. With an amplitude of about 

 three millimeters perfect contact is made at each vibration, and 

 the regularity of action, as shown by the steadiness of the spot 

 of light on the scale, is extremely satisfactory. The deflection 

 of the needle when the steady current is not balanced by the 

 intermittent current amounts in the case of the high fork to 

 125 scale divisions, using the one-tenth shunt; i.e., without 

 the shunt as it was used in practice to 1250 scale divisions. 

 With the fork's best action the resistances were adjusted until 

 closing the key would cause a deflection of less than half a 



