Maker's Name: 
Leeds & Northrup
Where made: 
19.5 × 29.5 × 14 cm

The resistance box has arms A, B and R. Each arm has a series of resistors, selectable by inserting a plug in the relevant hole on the arm. Each arm has a range of 1.0 Ohms to 10kOhms. Arms A and B are adjacent, and tha ratio A/B can be varied by suitable selection of resistors. Arm R is adjacent to the external unknown resistor X. With the bridge in balance, the value of the external resistor X is given by AR/B.