Signedplate on top panel: DE FOREST RADIO-TELEPHONE SYSTEM / VARIOMETER / TYPE RJ NO 30 / THE RADIO TELEPHONE CO., N.Y. U.S.A.
Inscribedplate on side: SERIAL NO. 165-09
FunctionVariometers are a form of variable inductance that was popular as a tuning coil for radio circuits 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 the 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.
This particular variometer makes the geometry of the device explicit as its scale has units of and angle rather than inductance.
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.
ProvenanceFrom the Physics Dept., Bowdon College, Brunswick, Maine. Collected in May 14, 1991. Courtesy of Prof. Elroy LaCasce.