(3) 
with their leads; p, and py the characteristie constants of their 
action on the needle (the differential quotients of the mutual 
induction coefficients by the rotations); C,; and C the capacities of 
the two condensers to be compared; ej and es the capacities of the 
non-inductive parts of the condensors connected to the electrometer ; 
then the equation for the equilibrium of the needle is!) 
Cy ) In Ca 2 
nn ORT wae 
If however this equation is fulfilled the equality of C, and C, does 
not immediately follow. We can first make pj =p; by scme known 
method. In this ease equation (1) takes the form 
2 C 2 
Ceo ce 
We can now reverse C, and Cy thus changing their influence and, 
keeping p constant, alter 7 through a capacity such that the equi- 
librium of the needle is not further aitered by this reversal. Hence 
besides (1') we have 
: 2 2 
te ah te 
And from (1') and (2) it follows that we must have y; = 72 and 
cj = cy if we wish to arrive at 
It would however be difficult ta make these quantities p and 7 
equal with the necessary accuracy. Further they alter with every 
change in the nullpoint of the electrometer needle, and hence these 
equalizations would have to be often repeated and would be certainly 
very tedious and lengthy. On account of these difficulties I have 
modified the method as follows. 
Cz remains permanently unaltered, C is an adjustable condenser, 
and is so arranged that the needle will not move when the coil is 
started. The condenser, of which the capacity is required, is now put 
in parallel with C,. To bring the needle back to zero, the capacity 
C, must be decreased by a measurable amount which is equal to 
the required capacity. In this way the symmetry of the electrometer 
ete. requires no attention. The sole condition is that the wires, which 
put the unknown capacity in parallel with C, have not themselves 
1) See MaxwerL Electricity and Magnetism. Vol. I. p. 219. 
