Radio Shack

Action of the Two Electrode Vacuum Tube.–Now in a vacuum tube
detector a wire filament, like that of an incandescent lamp, is
connected with a battery and this forms the hot element from which the
electrons are thrown off, and a metal plate with a terminal wire
secured to it is connected to the positive or carbon tap of a dry
battery; now connect the negative or zinc tap of this with one end of
a telephone receiver and the other end of this with the terminals of
the filament as shown at A in Fig. 71. If now you heat the filament
and hold the phone to your ear you can hear the current from the B
battery flowing through the circuit.
[Illustration: (A) and (B) Fig. 71.–How a Two Electrode Wrxv Mhz State College Tube Acts as
a Relay or a Detector.]
[Illustration: (C) Fig. 71.–Only the Positive Part of Oscillations
Goes through the Tube.]
Since the electrons are negative charges of electricity they are not
only thrown off by the hot wire but they are attracted by the positive
charged metal plate and when enough electrons pass, or flow, from the
hot wire to the plate they form a conducting path and so complete the
circuit which includes the filament, the plate and the B or
plate battery, when the current can then flow through it. As the
number of electrons that are thrown off by the filament is not great
and the voltage of the plate is not high the current that flows
between the filament and the plate is always quite small.
How the Two Electrode Tube Acts as a Detector.–As the action of a two
electrode tube as a detector [Footnote: The three electrode vacuum
tube has entirely taken the place of the two electrode type.] is
simpler than that of the three electrode vacuum tube we shall describe
it first. The two electrode vacuum tube was first made by Mr. Edison
when he was working on the incandescent lamp but that it would serve
as a detector of electric waves was discovered by Prof. Fleming, of
Oxford University, London. As a matter of fact, it is not really a
detector of electric waves, but it acts as: (1) a _rectifier_ of the
oscillations that are set up in the receiving circuits, that is, it
changes them into pulsating direct currents so that they will flow
through and affect a telephone receiver, and (2) it acts as a _relay_
and the feeble received oscillating current controls the larger direct
current from the B battery in very much the same way that a telegraph
relay does. This latter relay action will be explained when we come to
its operation as an amplifier.
We have just learned that when the stream of electrons flow from the
hot wire to the cold positive plate in the tube they form a conducting
path through which the battery current can flow. Now when the electric
oscillations surge through the closed oscillation circuit, which
includes the secondary of the tuning coil, the variable condenser, the
filament and the plate as shown at B in Fig. 71 the positive part of
them passes through the tube easily while the negative part cannot get
through, that is, the top, or positive, part of the wave-form remains
intact while the lower, or negative, part is cut off as shown in the
diagram at C. As the received oscillations are either broken up into
wave trains of audio frequency by the telegraph transmitter or are
modulated by a telephone transmitter they carry the larger impulses of
the direct current from the B battery along with them and these flow
through the headphones. This is the reason the vacuum tube amplifies
as well as detects.
How the Three Electrode Tube Acts as a Detector.–The vacuum tube as a
detector has been made very much more sensitive by the use of a third
electrode shown in Fig. 72. In this type of vacuum tube the third
electrode, or _grid_, is placed between the filament and the plate and
this controls the number of electrons flowing from the filament to the
plate; in passing between these two electrodes they have to go through
the holes formed by the grid wires.
[Illustration: (A) and (B) Fig. 72.–How the Positive and Negative
Voltages of Oscillations Act on the Electrons.]
[Illustration: (C) Fig. 72.–How the Three Electrode Tube Acts as a
Detector and Amplifier.]
[Illustration: (D) Fig. 72.–How the Oscillations Control the Flow of
the Battery Current through the Tube.]

It was the S O S of the international code, but in a new sort of
wireless–by pistol shots!
Trembling for the results that his desperate action might bring upon
them, Slim waited, bending now and then over the unconscious form of Tom
Rawle.
But in fifteen more minutes his inventive genius was rewarded. From a
considerable distance, but each time more distinctly, now came the
repeated call of “_Whip-poor-will_,” and in less time than it seemed
possible that they could make it, the other group had returned.
In low commands the lieutenant then directed affairs, and in exactly the
way that he had been carried out of the hold of the _Everett_ on the
verge of suffocation, so they carried poor Tom Rawle back to their own
lines.
And when he had been placed upon a cot in the first emergency hospital,
Lieutenant Mackinson hurried off to make his report, in the honor of
which all shared.
For not only had they found a location from which to wireless
advance-line communications to field headquarters, but they had also
intercepted a message, knowledge of which resulted in a quick change of
plans by which the Americans were able to beat the enemy at his own game
on the morrow.
“Rawle was suffering more from loss of blood than from any seriousness
of the injury itself,” the surgeon told them when they asked there of
their friends condition, on their way to their own quarters. “He will
be around all right again in a weeks time.”
And so, much desperate work accomplished on their first night within the
firing lines, the lads threw themselves upon their cots to dream of
spies and captured Germans and injured soldiers and calls for help by
new methods in wireless.
CHAPTER XIII
THE CAVE OF DEATH
It is one of the fortunes, or misfortunes, of war that a position gained
one day, even at great human sacrifice, may be of no real or practical
value whatever the next. So it was with the advance post of
communication located by Lieutenant Mackinson and his party under such
dangerous conditions during the night before.
The information which they had gained through tapping the enemys wire
enabled the American and French troops, operating together, to prevent
the German trick from being carried into effect. More than that, it
enabled them to turn the knowledge of those plans to such good advantage
that the allied brigades swept forward in terrible force against the
weakest points in the enemy line. They pushed the whole Boche front back
for more than a mile–at the very point where it had been considered
strongest!
As a consequence, the point of communication which Klan 93.5 Mhz In Glasgow the lieutenant and
his aides had established with so much difficulty was now well within
the territory held by the American and French fighters. The requirements
for a further advance now made it necessary to have another outpost
point of communication as near to the enemy trenches as the first one
was before the days battle put the Allies a mile further forward.
And so, except for Tom Rawle, who was resting easy from his hip wound,
the same party started out at the same tune for the same purpose on this
second night, but with a very much sharpened realization of the
obstacles they had to overcome and the chances they faced of being
wounded or captured.
“We take an entirely different direction,” Lieutenant Mackinson told
them, as he looked up from the map he had been studying. “We go to the
north and east and as close to the observation trenches as possible.”