5-6. A magnetized rod, about 4 inches long and 1/4 inch in diameter, is mounted near
each tone wheel. (See figures 5-1 and 5-2.) A small coil of wire is wound near one
end of the magnet. The tip of the magnet at the coil end is ground to a sharp edge
and mounted near the edge of the associated tone wheel. Each time that a tooth of
the wheel passes the rod, the magnetic circuit changes and a cycle of voltage is
induced in the coil. The voltage is very small and is of known frequency. The
frequency is predetermined by the number of teeth and the speed of the rotating
tone wheel. Larger coils are used with tone wheels of lower frequencies to provide
good low frequency output, but smaller coils are used with tone wheels of higher
frequency to prevent excessive losses.
5-7. Copper rings are mounted on certain low frequency coils for the purpose of
reducing harmonics. The eddy current loss in such a ring is small for the
fundamental frequency of the coil, but is high for its harmonics. As a result, the
the relative intensities of any harmonics which may be produced by irregularities in
the tone wheels are reduced.
5-8. The edge of each tone wheel and the tip of each magnet are coated with lacquer
to prevent corrosion, for, should oxidation set in, the change in tooth shape would
introduce undesirable frequencies.
5-9. Filters for eliminating spurious harmonics from the generated simple tones are
located on the top of the main tone generator, and consist of filter capacitors and
reactors. (See figures 5 & 6 in the Main Generator section.) (These capacitors and
reactors are tuned units and are called tone generator filters.)
5-10. The tone generator filters have a single tapped winding. This tap is grounded
and one side, which is connected to the associated coil assembly through a
capacitor, forms a resonant circuit for the fundamental frequency of that coil.
Harmonics are suppressed. The capacitors for frequencies 49 to 54 inclusive are
0.255 mf, and the capacitors for frequencies 55 to 91 inclusive are 0.105 mf. Both
capacitors and reactors are used with frequencies numbered 49 to 91 inclusive. On
frequencies 44 to 48 inclusive, the capacitors are omitted, but the reactors used
have a greater number of turns. Below frequency 44, neither capacitors nor reactors
are used; a length of resistance wire shunts each generator output. This resistance
wire is wound on the appropriate magnet coil.
5-11. The tone generator filters are mounted on top of the generator at an angle to
minimize reaction between them. Wires connect the filters to the coil assemblies and
to the terminal strip on the generator. Ninety-six terminals are provided on this
strip; 3 terminals are grounded to the generator frame and serve to ground the
manuals and pedals, and 91 terminals carry the various frequencies.
16
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