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This is a slide wire potentiometer, invented by Johann Christian Poggendorff in 1841, and produced by Leeds & Northrup somewhere around the 1950s.
This type of instrument was used in labs and universities to precisely measure voltages to within 0.00001 of a volt.
This unit is in good condition cosmetically, but does have some surface scratches near the large voltage knob. One of the pictures shows them, but it was hard to get a decent photo, because the surface is such a shiny beautiful black. I had long planned to thoroughly clean and polish it inside and out for display, but I just don’t have the time. This particular one has a clear plexiglass bottom, so you can look at the circuitry inside. I love the way the wiring is constructed, using heavy gauge bare copper, bent at precise angles. It looks like a work of art. There is no top, what you see in the photos is what you will get.
These potentiometers were used with an external standard cell and an external galvanometer.
At the heart of the potentiometer is a long slide wire. In the Type K potentiometer this was in the form of a ten-turn coil, the big drum on the right. To the right of that are the coarse, medium and fine current adjustments. The large knob near the middle is the coarse voltage adjustment, in steps of 0.1volts. The three buttons in from of it are low, medium and high sensitivity keys for the galvanometer. The knob at the back left is the fine control for standardising: It is set to the voltage from the standard cell, adjusted for temperature. The middle switch on the left is the range selector. The front switch on the left sets the function to measure or standardize. The back has connections for the unknown voltage, the galvanometer and both the reference and supply battery.
I only ship to the United States. If you are an international buyer, I will ship to the global shipping center, and they will then re-ship it to you.