Rectangular wooden case with sloped top; black instrument panel on top surface; instrument panel features six connection posts for the external generator, a galvanometer meter under glass, nine black plastic knobs, four of which are mounted on metal scales, and five other metal connection posts along the right side and bottom; above the instrument panel is a wooden lid fastened by two screws; underneath the lid is an empty storage area; a black metal handle is on the top side of the wooden case.
Impedance bridges compare electrical impedances, which may contain inductance, capacitance, and resistance. They use a method similar to that employed by Wheatstone bridges, which use a four-arm bridge circuit to compare an unknown resistance to that of a known standard.
The Type 650-A was introduced in 1933 and was popular based on its flexibility and utility. The 650-A was one of the best-selling instruments by the General Radio Co. and sold for the low cost of $175. This particular instrument was previously used at the BASF (Baden Aniline and Soda Factory) dye laboratory in Rensselaer, New York.
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