Plug type, Post Office Box in wooden polished  box with hinged lid , Interchangeable plugs with moulded in fluted plastic tops  and having rectangular brass metal blocks approx. 12mm hold precision cut, and are fitted on a black  thick insulating plate, ohms mentioned on plate. Resistance Coils of  Constantan wire double silk  covered  eureka  is wound non-inductively, and have a negligible temperature co-efficient. Two spring keys having special contact for battery and galvanometer. P.O. Box  resistance arms contains 16 coils from 1 to 5000  - ohms with infinity plug, In 3 ratio P.O box  two proportional arms each with coils of 10, 100 and 1000-ohms. &  in 4 ratio P.O box  two proportional arms each with coils of 1, 10, 100 and 1000 ohms.