Resistance Box


Description: The resistance coils that go to make up a resistance box of 75 or more years ago consist of wire of one of the abovementioned alloys, covered with silk or paraffined cotton, wound with great care and adjusted in length to have resistance of a definite number of ohms Each end of a coils is soldered to a brass piece, as in figure 1, the first coil is soldered to brass A and B whilst the second is soldered to brass B and C. The brass pieces are themselves fixed to a block of ebonite which forms the top of the resistance box. Sufficient room is left between each adjacent block to allow brass plugs to be inserted. While the plugs are in position current flows through the plug which shorts out the resistance. On taking the plug out, the current is forced through the resistance. By this means the amount of resistance thrown into the circuit can be controlled. A typical example is shown in figure 2. The series of resistance coils are chosen to allow any desired value of resistance to be inserted in a circuit. A typical box may contain coils with the following numbers of ohms: 1, 2, 2, 5, 10, 20, 20, 50, 100, 200, 200, 500, up to 10,000 in some boxes. Values less than 1 ohm are often included and other combinations will often be found. The above combination allows any value of resistance to be selected, for example, 456 ohms would be made up of 200 + 200 + 50 + 5 + 1 by taking out just 5 plugs.


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