Ratio of the Electromagnetic to the Electrostatic Unit. 319 



condenser is charged; when they approach, _p dips into the 

 other cup and the condenser is discharged. The points 

 must be at least half a millimetre above the surfaces of 

 the mercury when the fork is at rest, in order to avoid 



Fiar. 3. 



both dipping at once and short-circuiting the condenser. 

 With an amplitude of about three millimetres perfect con- 

 tact 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 deflexion 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 ; or, without the shunt, as it was 

 used in practice, to 1250 scale-divisions. With its best action 

 the resistances were adjusted until closing the key would 

 cause a deflexion of less than half a scale-division, corre- 

 sponding to less than l-2500th part of the whole current. 

 To obtain a regularity of action which permitted such 

 accurate observations required a very delicate adjustment of 

 the distances between the surfaces of the mercury and the 

 points above them, as well as clean surfaces and a steady 

 current. 



5. Battery. — About forty cells of a storage battery, with a 

 total electromotive force of about eighty volts, were used. A 

 higher electromotive force, at first proposed, was thought to 

 be unnecessary. 



6. Resistances. — The resistance a was taken from a box of 

 Elliott Bros., the total resistance of which was about 12,000 

 ohms ; the resistance d was a 100,000 ohm box from the same 

 firm. The first of these, box A, was carefully calibrated by 



