To make a tube set up powerful oscillations then
To make a tube set up powerful oscillations then, it is only necessary
that an oscillation circuit shall be provided which will feed part of
the oscillations set up by the tube back to the grid circuit and when
this is done the oscillations will keep on being amplified until the
tube reaches the limit of its output.
[Illustration: (C) Fig. 94.–How a Direct Current Sets up
Oscillations.]
The Operation of C. W. Telegraph Transmitters With Direct
Current–Short Distance C. W. Transmitter.–In the transmitter shown
in the wiring diagram in Fig. 76 the positive part of the 110 volt
direct current is carried down from the lamp socket through one side
of the panel cut-out, thence through the choke coil and to the plate
of the oscillator tube, when the latter is charged to the positive
sign. The negative part of the 110 volt direct current then flows down
the other wire to the filament so that there is a difference of
potential between the plate and the filament of 110 volts. Now when
the 6-volt battery current is switched on the filament is heated to
brilliancy, and the electrons thrown off by it form a conducting path
between it and the plate; the 110 volt current then flows from the
latter to the former.
Now follow the wiring from the plate over to the blocking condenser,
thence to _clip 3_ of the tuning coil, through the turns of the latter
to _clip 2_ and over to the filament and, when the latter is heated,
you have a _closed oscillation circuit_. The oscillations surging in
the latter set up other and like oscillations in the tuning coil
between the end of which is connected with the grid, the aerial and
the _clip 2_, and these surge through the circuit formed by this
portion of the coil, the grid condenser and the filament; this is the
amplifying circuit and it corresponds to the regenerative circuit of a
receiving set.
When oscillations are set up in it the grid is alternately charged to
the positive and negative signs. These reversals of voltage set up
stronger and ever stronger oscillations in the plate circuit as before
explained. Not only do the oscillations surge in the closed circuits
but they run to and fro on the aerial wire when their energy is
radiated in the form of electric waves. The oscillations are varied by
means of the telegraph key which is placed in the grid circuit as
shown in Fig. 76.
The Operation of the Key Circuit.–The effect in a C. W. transmitter
when a telegraph key is connected in series with a buzzer and a
battery and these are shunted around the condenser in the grid
circuit, is to rapidly change the wave form of the sustained
oscillations, and hence, the length of the waves that are sent out.
While no sound can be heard in the headphones at the receiving station
so long as the points of the University Of North Carolina Wunk-tv key are not in contact, when they are in
contact the oscillations are modulated and sounds are heard in the
headphones that correspond to the frequency of the buzzer in the key
circuit.
The Operation of C. W. Telegraph Transmitters with Direct
Current.–The chief differences between the long distance sets which
use a direct current, i.e., those described in Chapter XVI, and the
short distance transmitting sets are that the former use: (1) a
motor-generator set for changing the low voltage direct current into
high voltage direct current, and (2) a chopper in the key circuit. The
way the motor-generator changes the low- into high-voltage current has
been explained in Chapter XVI.