Quadrant-Electrometer. 293 
If 6 be small, we obtain 
dW A+B 
gt =a(A—BY0-=**), 
the formula in Maxwell. 
Returning now to our original equation, we have 
2W= gyA?+ qo2B? + qe3C? + GagD? 
—2q,,AB—q3;3AC—2¢,AD 
— ¢3BC—2q2,.BD 
+06(4—B)(o—4 3%) 
3 
2 
involving in all eight constants, g,, &c. being now regarded as 
representing the values of the coefficients in the zero position. 
Q= gquA— WB—$¢33;0—gquD+a0(C—A), ) 
= — Q12A+ 922B —4933C — Qo4D —a26(C—B), | 
Q3= —4933A—$93sB + Y33C + a6(A—B), 
s=— GuA— GB + 44D. ) 
‘We may now discuss a variety of important particular 
cases. 
(a) B is put to earth; A then is connected to a condenser, 
capacity a, charged to potential V: we want to know V from 
the reading of the electrometer. Here 
aV —$9330= (a+ qu JA—$G330 +26(C—A), 
VH=As Mar af(C—A) 
a a 
Neglecting A compared with C, and assuming 
@=(A-B)(C— 5 
peeve ve {14214 o}. 
The apparent capacity of A increases with C. 
(6) Bisagain zero. A is connected to a source, but is 
disconnected and insulated when the deflection of the needle 
is @’; the final deflection is 0: required the potential V of the 
ane gu V — 39330 + a0/(C—V) 
= GyA—$q33C +a6(C—A), 
V=A+ a(@—@')C 
qu a! 
ar(1—5) ee 
Sate saan |. 
Gi 
