Signedmarked on plate above dial: AMRAD / Variometer 2611 / American Radio and Research Corporation, MEDFORD HILLSIDE, MASS.
FunctionVariometers a radio tuning device, a form of variable inductance. It was very popular in the 1920s.
Tuning coils in general are used to alter the transmission or reception frequency of a circuit. The frequency is inversely proportional to the square root of a circuit's capacitance and inductance, so changing the circuit's inductance is a way to alter its transmission or reception frequency.
Variometers in particular consist of two coils, one inside the other. The internal coil can be rotated so that its axis can be parallel, perpendicular, or opposite (as well as intermediate settings) to the axis of the external coil. The current circulates first inside one coil, then the other. As a result, the inductance will be at a maximum when both coils are parallel, and at a minimum when they are in contrary directions. This allows the circuit's frequency to be varied within a range.
One of the advantages of variometers compared to other forms of tuning coil is that the total length of wire in the circuit remains the same, so its total resistance remains constant.
Primary SourcesA different device made by the same company is described in, James R. Cameron, Radio For Beginners: An Elementary Text Book Written So That The Amateur Can Understand It. New York City, NY: The Technical Book Company, 1922. p.84.
Available online in Google Books here.
ProvenanceFrom the Bowdoin College Department of Physics, Brunswick, ME. 1998.