Signedplate on rheostat knob: TRADE OHMITE MARK / REG. U.S. PAT. OFF. / VITREOUS ENAMELED / RHEOSTAT-POTENTIOMETER / PAT. NOS. RE-19607-2040278 / OTHER PATENTS PENDING / OHMITE MFG. CO. / CHICAGO, ILL. / MADE IN / UNITED STATES OF AMERICA
etched and white filled on top of condenser: VARIABLE AIR CONDENSER / TYPE I.P.300 SERIAL No. 519 / CAPACITY .005 MF.D. / WIRELESS SPECIALTY APPARATUS CO. / BOSTON, U.S.A.
Historical AttributesThis Harvard teaching model was made by the Department of Physics in order to demonstrate the essential components of an electronic circuit: a resistor (R), inductor (L), and capacitor (C). These are also importnat in understanding how radio reception works.
A radio antenna picks up all waves in the air. The receiver must filter out what one doesn’t want and enhance what one does. To select the desired carrier wave, radios have a resonant circuit that can be tuned to match its frequency. The typical resonant circuit combines a coil and a capacitor. Often the capacitor is the component adjusted in the tuning process, but a coil may also be used.
The resistor, inductor, and capacitor in this teaching model are types found in early radios. They are:
R Rheostat: A variable resistor used to control
large currents in a circuit.
L Variometer: A pair of coils wired in series so
that moving or rotating one with respect to the
other adds or subtracts to the total inductance.
This assists in tuning the radio.
C Variable air condenser: a set of metal plates
that rotate with respect to fixed plates in order to
vary the capacitance in the resonant circuit. This
also assists in tuning.
ProvenanceDepartment of Physics, Harvard University.