Signedetched and white filled on dial: GENERAL RADIO CO. / CAMBRIDGE, MASS
Inscribedon plate on side: HARVARD UNIVERSITY / 1511
in white on side of base: T.I.4
carved on base: PROPERTY OF / HARVARD ENGINEERING SCHOOL;
on paper label affixed to frame: This variometer is wound with wire made up of 10 strands of #32 S.C.C. wire the rotary and stationary coils each having 60 turns. With the coils in series, the carrying capacity os one amperes, and the inductance is approximately /9 / millihenrys at 0 and 2.0 millihenrys at 100. With the coils in parallel, the carrying capacity is doubled, and the inductance is reduced to about one quarter. This instrument should not be used where there will be a potential sufficiant to jump more then 1/8 accross its terminals. The rotary coil terminals are stamped "R" and the stationary "S" Series Connection. (diagram) (diagram) Parallel Connection / Variometer. General Radio Co., Cambridge, Mass.
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.
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.
Historical AttributesUsed for instruction at the Engineering School.
ProvenanceFrom the Cruft Laboratory, Harvard University.