ALTERNATING-CURRENT SUSCEPTIBILITY METER 151 
The indicating device consists of a pointer type, alternating-current 
galvanometer, energized by a three-stage, resistance-capacitance-coupled, 
vacuum-tube amplifier, which is provided with an appropriate output circuit 
that is designed to prevent the flow of direct current through the galvanom- 
eter circuit. The shunt 73 permits critical damping of the galvanometer, 
and also furnishes a convenient means for adjusting its sensitivity. An ex- 
ternal storage battery, B;, supplies filament power for the amplifier, while the 
grid-bias battery, Bo, and plate battery, B3, are included within the instru- 
ment case. 
ae, 
i tH 
eo ac. 
B, Bz Bs 

Fig. 2. Electric circuits of the Susceptimeter. 
The switch S, makes it possible to energize simultaneously the bridge and 
galvanometer field winding with 60-cycle alternating current, or 6-volt direct 
current, while 3 is for the purpose of connecting the galvanometer directly 
to the output junctions of the bridge, or including the amplifier in the in- 
dicator circuit. The switches S; and Ss control the external alternating-cur- 
rent and direct-current power sources, respectively. 
THEORY 
Let R; and R, be the resistances of the ratio arms; R; the resistance of the 
arm containing the inductances L; and L2; R, the resistance of the arm con- 
taining the inductances L; and L,; thea, the condition imposed for a balance 
with direct current is R,R3 = RR, and.by varying 7; a balance can be obtained 
for steady currents. 19 
