282

ELECTRICAL APPARATUS

II ',

with the frequency of the exciting current, but of a power, as
many times greater, as the machine output is greater than the
exciting power.
.Thus such an inductor alternator with alternating-current
excitation can be used as amplifier. This obviously applies
equally much to the other types, as shown in Figs. 136, 137
and 138. ?
Suppose now the exciting current is a telephone or micro-
phone current, the rectified generated current then pulsates with
the frequencies of the telephone current, and the machine is a
telephonic amplifier.
Thus, by exciting the high-frequency alternator in Fig. 138,
by a telephone current, we get a high-frequency current, of an
amplitude, pulsating with the telephone current, but of many
times greater power than the original telephone current. This
high-frequency current, being of the frequency suitable for radio
communication, now is sent into the wireless sending antennae,
and the current received from the wireless receiving antennae,
rectified, gives wireless telephonic communication? As seen,
the power, which hereby is sent out from the wireless antennse,
is not the insignificant power of the telephone current, but is the
high-frequency power generated by the alternator with telephonic
excitation, and may be many kilowatts, thus permitting long-
distance radio telephony.
It is obvious, that the high inductance of the field coil, F, of
the machine, Fig. 138, would make it impossible to force a tele-
phone current through it, but the telephonic exciting current
would be sent through the armature winding, which is of very
low inductance, and by the use of the capacity the armature
made self-exciting by leading current.
Instead of sending the high-frequency machine current, which
pulsates in amplitude with telephonic frequency, through radio
transmission and rectifying the receiving current, we can rectify
directly the generated machine current and so get a current
pulsating with the telephonic frequency, that is, get a greatly
amplified telephone current, and send this into telephone circuits
for long-distance telephony.
162. Suppose, now, in the inductor alternator, Fig. 139, .with
low-frequency alternating-current excitation, giving a voltage
wave shown in Fig. 140, we use several alternators excited by
low-frequency currents of different phases, or instead of a single-