Commonly used in EE
labs, the decade resistor box consists of a case, two terminal
s, and (usually) 28 switch
The switches are labeled from 1M to 1, from left to right, decreasing by a factor of 10 each time, hence decade. Going from top to bottom, there are four rows of switches, 1 through 4. By asserting switches, the resistance between the terminals changes, such that it adds up to the total of the numbers on the asserted switches.
This is an extremely useful device, especially in a lab setting, because the decade resistor box can be wired into a circuit, and then have the value of its resistance changed, to allow the researcher or student to see the effects of varying resistances at that point in the circuit.