Resistance Box by Philip Harris [vintage]002917
A resistance box or resistor substitution box is a unit containing a number of precision resistors connected to panel terminals or contacts so that a desired resistance value can be obtained by withdrawing plugs (as in a post-office bridge) or by setting multicontact switches, which allow any one of various discrete resistances offered by the box to be dialled in. Usually the resistance is accurate to high precision, ranging from laboratory/calibration grade accuracy of 20 parts per million, to field grade at 1%. Inexpensive boxes with lesser accuracy are also available. All types offer a convenient way of selecting and quickly changing a resistance in laboratory, experimental and development work without needing to attach resistors one by one, or even stock each value. The range of resistance provided, the maximum resolution, and the accuracy characterize the box. Values: 1, 2, 2, 5, 10, 20, 20, 50, 100, infinity.