Measuring the Electrical Capacity of a Condenser. 101 



any electrical absorption in the condenser, its effects will 

 depend on the time during which the contact lasts. The 

 experiments to be described were undertaken with the view 

 of testing the method for these defects. The time during 

 which the plates of the condensers are connected with the 

 battery is the time during which the spring P N is in contact 

 with 6. This is only a fraction of the period of the tuning- 

 fork which governs the commutator, a fraction which depends 

 on the distance between the end of the screw T and the point 

 G, and on the strength of the current in the electromagnet. 



A series of experiments were made to test the effect of 

 varying the position of the screw T, keeping the battery the 

 same. In these, four Leclanche cells were used in the battery- 

 circuit A B, and two pint Groves to drive the commutator. 

 The position of the spot of light on the scale was noted, and 

 then the value of the resistance d adjusted until no deflection 

 was produced when the galvanometer-circuit was closed by 

 means of a key. 



The commutator, when working correctly, emitted a defi- 

 nite sound which was readily recognized; and it was found 

 that the screw T could be adjusted within certain limits without 

 altering this sound. 



Table I. gives the results of these observations. 



Table I. 



Commutator adjustments. Ba" units 



Screw T set , 905 



Screw T readjusted, note good 906 



n ii ii ii JOo 



„ „ „ bad e 906 



ii ii ii good 906 



Screw T loosened one turn. Spot off scale. 



Screw T readjusted, note good 906 



Thus the value of d required to give a balance is prac- 

 tically the same throughout, and the commutator with a defi- 

 nite battery-power can readily be set to give a definite 

 result ; moreover, a balance could never be obtained except 

 with the resistance 905 or 906. In the fourth observation 

 the note of the commutator was marked as bad ; but the spot 

 remained stationary, showing that though there seemed to be 

 a slight irregularity in the sound, it was too small to affect 

 the balance. The sensitiveness was such that an alteration in 

 d of 1 B.A. unit produced a deflection of from two to three 

 scale-divisions. The number of vibrations of the fork was 

 about 32 per second. 



