Black finished cylindrical casing with three copper electrodes in a line next to product inscription; nickel knob is raised above the cylinder; outer dial goes from 0-15 (but only three digits are visible through its viewing slit); outer dial value changes when inner dial (0-100) is manually rotated 360 degrees; knob handle is black plastic.
Potentiometers are used to measure electromotive force (emf) by balancing it with a known emf. The concept was first developed in 1841 by J. C. Poggendorff and improved throughout the 19th century by Latimer Clark and J. A. Fleming.
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