Each tone wheel is a steel disc about 2 inches in diameter, accurately
machined with a definite number of high and low points on its edge
(See Figure 3). Each high point on a tone wheel is called a tooth. The
number of teeth on each of these tone or phonic wheels, in conjunction
with the speed at which the tone wheel is revolving, determines the frequency
of the tone generated.
Each driving gear, with its two bakelite gears and four tone wheels, runs in a
separate compartment magnetically shielded from the rest by steel plates which
divide the generator into a series of bins.
All four tone wheels in any one compartment run at the same speed. The
individual tone wheel shafts are mounted in bearings made of a special
porous bronze and each of these bearings is connected to the oiling system
by a cotton thread from the oil trough. Thus, oil from the trough is carried
by capillary action to all bearings, penetrating them and lubricating the
actual bearing surface. The drive shaft and both motors are lubricated in
a similar manner. It is very important to use the recommended grade of
oil regularly, as it is essential to the proper operation of the organ that the
generator be well lubricated. If oil of varying grades is used, it is likely
that the generator may be sluggish in starting, and in time the threads may
gum up and prevent the proper flow of oil.
The two spring couplings on the motor shaft, the flexible couplings
between sections of the drive shaft, and the tone wheel spring couplings
all contribute to absorbing variations in motor speed. The synchronous
motor does not deliver absolutely steady power. but rather operates
with a series of pulsations, one with each half cycle. If the tone wheels
were rigidly coupled to the motor, this slight irregularity would carry
extra frequencies into each tone wheel. In addition, hunting is suppressed
by the resilient couplings and inertia members of the synchronous motor
proper.
Associated with each tone wheel is a magnetized rod about 1/4 of an inch
in diameter and 4 inches in length, with a coil of wire wound near one end
(See figure 3). The tip of the magnet at the coil end is ground to a sharp
edge and mounted near the edge of the tone wheel. Each time a tooth passes
this rod it causes a change in the magnetic field which induces a small
voltage in the coil. the frequency being determined by the number of teeth
and the wheel speed.
Small coils are used on the higher frequency magnets and larger coils
on the lower frequencies. It is found that large pole pieces are needed on
the low frequency magnets to give good frequency output. but it is necessary
to use smaller ones on the high frequencies to prevent excessive iron losses.
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