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A ceramic resistance wire rheostat
Concealed resistance wire
A ceramic carbon pile rheostat, single type

A Rheostat is used in a foot control or knee press controller to regulate the speed of the motor, it works by limiting the amount of current to the motor, light pressure on the foot control creates a high resistance and the motor will run slowly. As the foot control is depressed the resistance is lowered and the motor speeds up.

For more information see Adjusting & Repairing a Rheostat in Conserving, Restoring & Repairing