SM003     Resistance Box      circa 1965     Electricity

In order to obtain specific values of resistance to a high standard of accuracy in electrical experiments a resistance box is used.   This consists of a number of fixed wire-wound resistances of a series of values from which the required resistance value is chosen by shorting out the values that are not required.   This shorting is done with contact keys that are removed to introduce the desired value corresponding to the key.   The series of resistances is chosen so that, for example, any value from zero ohms up to 1110 ohms can be obtained in steps of one ohm.  

The resistances are specially wound from cotton-covered Constantan wire so as to have a low coefficient of temperature and also low inductance.   The accuracy of each resistance is of one part in a thousand.

The connections to the resistance box are heavy binding posts and an infinity key to open the circuit is provided.