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452
Radio News for November, 1928
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ALTERNATING CURRENT
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ONE CYCLE:
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FIG.6 QA
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of the variation of the length of the rod.
The rod will vibrate now with the same frequency
as the alternating current. Those
two cases are illustrated in Figs. 6A and
6B.
We will now return to the oscillator. To
simplify the case, we will assume that there
is no tuning condenser across the coils.
(Such an oscillator may operate provided
the amplifying power of the tube is high
enough. A single tube may be replaced by
an amplifier having several resistance -
coupled stages.) One coil is then inserted
into the grid circuit of the first stage, while
the other is connected to the plate of the
last stage. (One form of Professor Pierce's
patent application.)
Suppose, now, that the rod is at rest and
that the plate current has a constant intensity,
which is determined by the characteristics
of the tube and the applied volt-
ages. The rod becomes magnetized under
the influence of the field created by the
second coil. This magnetization is not uniform,
as the field strength along the rod is
not uniform.
Should there happen a sudden variation
of the plate current (due, for instance, to
an exterior cause) a tendency in the rod to
expand or contract, due to the Joule effect,
will be caused instantly. The situation is
somewhat similar to the case in which a
shock is given to the rod in the line of its
length. The particles of the metal are
thrown out of balance, and longitudinal
oscillation will take place in the rod under
the action of the two forces of elasticity
and inertia.
FREQUENCY OF THE ROD
The fundamental frequency of the rod
may be found by dividing the velocity of
sound in the substance of which the rod is
composed, by twice its length. During the
longitudinal oscillation of the rod, variation
in its magnetization will occur (V Mardi effect).
An oscillatory electromotive force
will be created in each of the two coils;
those electromotive forces will have the same
fundamental frequency although, as a general
rule, they will differ in their other elements.
A variation of the plate circuit then will
cause the appearance of two electromotive
dorces in the grid coil; one due to the mutual
ONE CYCLE ¡
A
Fig. 6.4 illustrates the
Tonle effect. For each
complete cycle of the magnetising
current, the rod
within the magnetic field
of that current vibrates
TWICE: that is, the frequency
of vibration of
the rod is equal to twice
the frequency of the al
tcrnating current. If, as
in Fig. 6B, a direct current
is added to the alternating
one, the rod will
vibrate at the frequency
of the alternating current.
I-PURE A.C.
II'RESULTING CURRENT
A
inductance between the two coils, and the
other as a result of the Joule effect and its
opposite. The resulting emf. will cause the
plate current to fluctuate with the natural
frequency of the rod. Ordinarily, the vibrations
of the rod produced by a shock, or any
other momentary disturbance in the internal
balancing forces, would not continue indefinitely;
as they are damped by the losses
of energy and would die out very quickly.
But here the conditions are different; if the
elements are properly selected and the circuit
is connected in the right way, the decrease
in the amplitude after each oscillation may
be compensated by the current in the tube,
and a sustained longitudinal oscillation will
take place in the rod. The rod will be driven
by the tube. (Something similar we find in
the oscillation of a pendulum maintained by
the driving force of a weight or a spring, as
in a dock; the energy of the pendulum after
each complete cycle is restored to its original
value by a release of corresponding
amount from the driving mechanism.)
Simultaneously with the vibration of the
rod, an oscillatory current, the frequency of
which is controlled by the rod, will flow
through the plate coil.
Tuning one of the coils with a condenser
or both together (Hartley circuit) does not
alter the fundamental principle of operation
in this type of oscillator.
SELECTION OF APPARATUS
The above simplified theory gives us some
ideas about the elements to be used and
their constants and characteristics.
(1) The tube should be of the "high -mu"
type, although satisfactory results may be -
obtained with other types.
(2) The substance of the rod is of the
highest importance. It must have sharply -
pronounced magneto -strictive properties
and, preferably, should not reverse the direction
of the change in its length during a
single half -cycle of the magnetizing force.
Comparing with Fig. 2, the characteristics
of iron, cast cobalt and nickel, we find that
the latter is the most suitable material of
the three.
Some of the nickel alloys, such as invar
(30% nickel, 63.8% iron and 0.2% carbon),
nichrome (60% nickel, 12% chromium, 26%
iron), or monel metal (66% nickel, 33.5%
copper and 0.5% iron), have been found to
require excessively long rods. For the purpose
of obtaining low frequencies, the ends
of a shorter rod may be loaded with weights.
Also, we may use a tube made of a magneto-
strictive substance which is filled out
with a metal having a low sound -velocity;
for instance, lead (4,025 feet per second, as
compared to 16,315 for nickel).
(3) As to the shape and the inductance
values of the coils, more complicated and,
to a certain extent, contradictory, considerations
enter. The plate coil has to be designed
in accordance with the direct -current
component of the peak current and the mag-
The complete magneto- striction oscillator as constructed in the RADIO News Laboratory.
The "loatd speaker" resting on the vibrating rod is merely a rolled sheet of
writing paper. Note the simplicity of the whole oscillator.