P=R3=consists of std. decade dial of 3 dials- 10x10, 100x10 & 1000x10 W

Q=R4=consists of standard decade dial of 3 dials- 10x10, 100x10 & 1000x10 W

R2 =consists of standard decade dial of 3 dials-.01x10, 0.1x10 & 1x10  W

L1 =single dial Inductance Box (10mH to 100mH) is to be connected from outside.

R2 =is connected in Series with L1  to from 3rd arm denoted as R

L 2 =unknown Inductance whose value is to calculated is connected in the 4th arm S

Terminals for oscillator, known inductance  L1 & L2  , & Headphone.