Q = R3  = Four Dial resistance box 0.1x10, 1x10, 10x10, 100x10ohms.

C1 = C2 = Single dial condenser Box of 0.1x10mfd.

R2 = A variable 4-dial resistance box of 1x10, 10x10, 100x10 & 1000x10ohms.

R1= Plug type resistance box 1 to 500 ohms.

L1= Unknown Inductance whose value is to be calculated

R1 is connected in series with L1  to form 1st arm P.  R2  is connected in series with C2  to from 4th arm S.  

Terminal for Headphone, Unknown Inductance, Oscillator.