620 PROFESSOR CHRYSTAL ON THE DIFFERENTIAL TELEPHONE. 
From these equations we get 
(M’D?+Q’D) 4 
MD? + QD + y—-KD%-E-F , 
1 
/T)2 , 
MD?+QD+¥ 
7 2 7 a 
(N'D?+K’D) 5 
ND? + RD + z—-KD%y=E-F. 
i 
7 2) 7 = 
N’D +tRD+5 
The condition y=z gives 
{(M—N)D + (QR), {MD*+QD+ x} JND?+RD +91 
1 ; j ; ; 1 1 7 ; ; , 1 
+x/MD +Q LN D?+RD+y}—y{N D+R i MD?+QD+x}=Q. 
Equating to zero coefficients of D, we get 
M—-N=0, Q-R=0, 
MN(x-y)=0, 
(W'R’ + W'Q) (g-4)=0, 
(M’ —N')gy+ QB (g—y)=0, 
nh. Te 
The last four equations require that 
X=Y ,M=N, OEE. 
And conversely, if we attach a condenser by means of very thick electrodes 
to one circuit of a differential telephone, and attempt to balance it by attaching 
similarly a condenser to the other circuit, the capacities of the condensers must 
be equal, the resistance and self-induction included between the armatures 
must be the same on both sides, and the resistance and self-induction of the 
remainder of each circuit must be the same. Here, therefore, five independent 
adjustments are necessary to secure silence in the differential telephone for all 
frequencies of the varying electromotive force. 
