The 4-Ratio P.O. Box has sixteen coils as above and two proportional arms each with coils of 1, 10, 100, 1000, ohms.
Resistance between 0.1 and 1111000 ohms can be accurately measured using a DC battery.
Two spring keys having good contact for battery and galvanometer is also provided.
The whole system is contained in a superior quality wood box.
The 3-Ratio P.O. Box has sixteen non-inductively would coils 1, 2, 2, 5, 10, 20, 20, 50, 100, 200, 200, 500, 1000, 2000, 2000, 5000 ohms and two proportional arms each with coils of 10, 100, 1000, ohms.
Also incorporates infinity gap.

Available in 3-Ratio and 4-Ratio.
P.O.Box 3-Ratio and 4-Ratio (Manganin coil) consists of 12mm & 14mm thick rectangular brass block and have accuracy ±0.05%.
P.O.Box 3-Ratio and 4-Ratio (Constantan coil) consists of 12mm & 14mm thick rectangular brass block and have accuracy ±0.1%.

Physics School Lab Equipment Suppliers in India 

Ltekonline are the manufacturers of Post Office Box Plug Type Electronic and Electric Lab used to demonstrate engineering principles in Engineering Colleges, Engineering Schools and Physics Lab Experiments for Engineering Teaching Lab in Universities