ï~~The Alternators of the Teiharmonium, 1906
Reynold Weidenaar
New York University Department of Film and Television
721 Broadway, 9th Floor, New York, NY 10003-6807
Thaddeus Cahill's Telharmonium, as completed in the Spring of 1906
at Holyoke, Mass., generated tones by means of power alternators.
Their construction and deployment are described, showing how
they produced both equal temperament and just intonation. Of special interest is the "phantom" octave, a 6th octave coaxed from a 5 -octave range of alternators.
Construction of the Alternators
Thaddeus Cahill began constructing
his full-sized commercial Telharmonium, for the generation and distribution of electrical music, in 1902.
He first demonstrated the instrument
to the public and the press in March,
1906. The tones were produced by
alternating current dynamos of the
inductor variety. As an iron tooth
moved past an electromagnetic field,
it generated an alternating current in
the armature of the inductor. The
stator was constructed to carry both
the electromagnetic field windings, or
coils, and the armature windings. The
number of north and south pole
pieces in the stator corresponded to
the number of rotor teeth. Since the
pole pieces were interspersed with
armatures, the stators had to be cut
with twice as many teeth as the rotors. The pole pieces were maintained in a state of constant excitation
by a 185-hp constant-speed dc motor,
which also rotated the pitch shafts
bearing the massive alternators. As
one rotor tooth would approach and
recede from the stator, a current
wave would be induced in the opposite armature coil. The current
would rise continuously from zero to
maximum positive value, then fall
through zero to maximum negative
value, and finally rise again to zero.
Consequently, a sine wave would be
induced in the armature coil. All the
armature windings were in series, so
the sum of currents generated at all
the teeth in the alternator would be
available at the output.'
In order to produce a sine
wave, the teeth of the inductors had
to be most carefully curved. This was
common practice in the construction
of alternators, but was difficult to
achieve in the fabrication of the
high-pitched units with their numerous small teeth. After cutting the
teeth of an alternator, Cahill would
run it and measure its wave shape.
He could then further mill the teeth
to attain more nearly the exact voltage curve desired.2 He also found
that simpler dish alternators, in
which the armatures and pole-pieces
(instead of teeth) were circumferentially mounted on rotating disks, were
preferable to the toothed-cylinder
type of inductor to produce the lowest fundamentals.3
Deployment of the Alternators
In the original patent design, there
were 12 pitch shafts tuned to the
equal-tempered scale. Each was to
carry 7 fundamental alternators, 6
3rd-partial alternators, and 5 5thpartial alternators. The frequency
relationships are shown in Table 1.4
All the alternators on a shaft were to
be tuned in just intonation. Each key
would have up to 10 separate electrical contacts for the various harmonics of its note, to connect the keys to
various alternators, all on the same
pitch shaft. The pitch of a single al
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