The ratio arm is two dials of 1, 10, 100 and 1000 ohm each.  The rheostat arm is in three dials having decade of 0.1, 1 and 10. A slide having 0.17ohm resistance is connected in series with unknown L. The slide wire is calibrated into 170 equal parts. A special double key is fitted to bridge.  The key is internally connected with terminals for connecting battery and galvanometer.  The bridge can also be used as Wheatstone bridge.