166 Mr. A. Campbell on the Use of 
9. Measurement of Capacities (Method E). 
As in fig. 11, let the primary P of the variable standard 
be connected in series with a condenser K to a periodic 
source H, while the secondary Q is connected to a vibration 
galvanometer as shown. Let M be adjusted until the galvano- 
meter gives no deflexion, in which case we have 
±Mai=± u L (U) 
or i/MK=-l. 
The secondary must be connected in such a direction that 
the lower sign is taken, and thus 
j*MK»l, (15) 
where M is in henries and K in farads. 
It is interesting to notice that this formula is similar in form 
to that for resonance with a condenser and self inductance, 
viz. jt> 2 LK=l. With self inductance we usually tune to get 
a maximum current through the galvanometer ; with mutual 
inductance, on the other hand, we tune for zero current. 
If M is in henries and K in microfarads, equation (15) gives 
159*2 
*^yHS' (16 > 
and if M and K are known, the frequency can be determined. 
If the frequency is not fairly high, the product MK 
becomes inconveniently high, e. q. M = l henry, K = 10 mfds. 
gives n = 50 -^ per sec. In contrast, n = 10,000 for 256 
microhenries and 1 mfd. 
The following is a practical example of the method : — The 
source was a microphone hummer for 2000 ~ per sec. and 
the detecting instrument a tuned telephone. A balance was 
