find the power expended in the plate circuit of a tube by
multiplying the number of volts in its battery by the number of amperes
which flows. Suppose the battery is 250 volts and the current 0.02
amperes, then the power is 5 watts. The "watt" is the unit for measuring
power. Tubes are rated by the number of watts which can be safely
expended in them. You might ask, when you buy an audion, what is a safe
rating for it. The question will not be an important one, however,
unless you are to set up a transmitting set since a detector is usually
operated with such small plate-voltage as not to have expended in it an
amount of power dangerous to its life.

In recent transmitting sets the tubes are used in parallel for the
reasons I have just told, but a different method of modulation is used.
The generation of the radio-frequency current is by large-powered tubes
which are operated with high voltages in their plate circuits. The
output of these oscillators is supplied to the antenna. The intensity of
the oscillations of the current in these tubes is controlled by changing
the voltage applied in their plate circuits. You can see from Fig. 123
that if the plate voltage is changed the strength of the alternating
current is changed accordingly. It is the method used in changing the
voltage which is particularly interesting.

[Illustration: Fig 123]

The high voltages which are used in the plate circuits of these
high-powered audions are obtained from generators instead of batteries.
You remember from Letter 20 that an e. m. f. is induced in a coil when
the coil and a magnet are suddenly changed in their positions, one being
turned with reference to the other. A generator is a machine for turning
a coil so that a magnet is always inducing an e. m. f. in it. It is
formed by an armature carrying coils and by strong electromagnets. The
machine can be driven by a steam or gas engine, by a water wheel, or by
an electric motor. Generators are designed either to give steady streams
of electrons, that is for d-c currents, or to act as alternators.

[Illustration: Fig 124]

Suppose we have, as shown in Fig. 124, a d-c generator supplying
current to a vacuum tube oscillator. The current from the generator
passes through an iron-cored choke coil, marked _L_{a}_ in the figure.
Between this coil and the plate circuit we connect across the line a
telephone transmitter. To make a system which will work efficiently we
shall have to suppose t