R1= R2= consists of ratio dial of 1, 10m 100 & 10000 ohms

R1=  r2 = consists of 3 dial resistance box of 1x10, 10x10 & 100x10 

C2 = consists of single dial capacitance box 0.1x10mgd.

C1 = unknown condenser whose value to be calculated

R1 = is connected in series with C1 to form 3rd arm ad.

R2= is connected in series with C2 to form 4th arm ab.

Terminals for connection of headphone, unknown condenser & oscillator