Fitted with rectangular brass blocks lacquered interchangeable plugs are connected with double nut system. Three ratio P.O. box has 16 non inductively wound coils and two proportional arms each with coils of 10,100 and 1000 ohms while 4 ration P.O box has 16 coils and 2 proportional arms each with coils of 1,10,100,1000 ohms. Two spring key for battery and galvanometer are provided. Fitted in teak wood box. Accuracy ±0.05 to 0.1%.
| Manganin Coil | Constantan Coil |
| A | B |
(i) | Three ratio coils | Three ratio coils |
(ii) | Four ratio coils | Four ratio coils |
