He flashed his light about the hole. It was difficult to tell where the
opening had been.
“Joe and Frank Hoskins!” cried Jerry, a new terror in his voice. “I
heard Joe shriek!”
Slim, catching his meaning, snatched a rifle from beside one of the
bodies, and with the butt of it began pounding Mmk License Llc Wnky frantically upon the side
of the cave where the entrance had been.
There was no answering knock.
“Joe,” shouted Jerry in a frenzied tone. “Joe! Can you hear me?”
No answer came, either from Joe or Frank.
“Pinned under tons of that stuff,” gasped Slim, the words trembling upon
his lips and a tear trickling down his cheek.
“I do not think so,” the lieutenant assured them. “Both Joe and Frank
were upon the outside when we entered.”
“But they would try to get us out,” said Jerry. “If they were out there
they would give us some sort of signal that they were trying to help
us.”
“We might not be able to hear them,” answered the lieutenant, even
against his own judgment. “But look at it this way. Even though they
never were inside here, they had a fair idea of what the place was like.
They knew from that that we needed help, and needed it quickly. If one
went alone, and anything happened to him on the way, the other might
wait here indefinitely, not knowing whether he had got assistance or
not. By going together they took the safest course.”
And Lieutenant Mackinsons reasoning was correct. That was exactly the
way Joe and Frank had figured it out, and, the latter forgetting all
about his own wound, they had started as fast as they could for the
American front.
“Keep cool, conserve your energy, and I feel certain everything will be
all right,” the lieutenant told the two friends with whom, in such a
short time, he already had gone through so many harrowing experiences.
He then gave each of the boys a pass, and told them to be aboard the
_Everett_ not later than half-past ten oclock, and departed for the
special work to which he had been called.
“Wouldnt you like to be a lieutenant, though?” exclaimed Joe
enthusiastically. “Just imagine being called from ship to ship to help
them out of their difficulties.”
And, discussing their aspirations and what the future held for them, the
three young men from Brighton went to mess, afterward brushed their
brand-new uniforms of the last possible speck of dust, and left the navy
yard for a stroll through the southern section of the city founded by
William Penn.
How far they walked none of them knew. They had turned many corners, and
their conversation had covered a wide field–always, however, turning
upon some military subject–when a church clock tolled out nine times.
“I think we had better return,” said Slim, who was beginning to tire
under the long days strain and excitement.
“Yes,” agreed Jerry, “but which way do we go?”
They were, in truth, lost. Uniformed as they were, they were ashamed to
ask directions, and finally agreed that Joe was right in indicating that
they should walk straight southward.
Twelve blocks southward they walked, and the damp, marshy atmosphere
assured them that they were nearing the river, but their only hope now,
as they plodded across desolate and deserted dumps, and even invaded a
truck patch or two, was that they would strike a road that led around to
the Kafm Mhz Grand Junction navy yard entrance.
“Whats that?” exclaimed Jerry in a hoarse whisper, grasping a boy on
either side of him by the arm. “Did you hear?”
“I thought I heard something,” averred Slim, also lowering his voice.
“What did it sound like to you?”
“We are almost upon the river bank,” said Joe. “It was someone rowing,
but it sounded to me as though they were using muffled oars.”
CHAPTER XV
OPERATION OF VACUUM TUBE RECEPTORS
From the foregoing chapters you have seen that the vacuum tube can be
used either as a _detector_ or an _amplifier_ or as a _generator_ of
electric oscillations, as in the case of the heterodyne receiving set.
To understand how a vacuum tube acts as a detector and as an amplifier
you must first know what _electrons_ are. The way in which the vacuum
tube sets up sustained oscillations will be explained in Chapter XVIII
in connection with the _Operation of Vacuum Tube Transmitters_.
What Electrons Are.–Science teaches us that masses of matter are made
up of _molecules_, that each of these is made up of _atoms_, and each
of these, in turn, is made up of a central core of positive particles
of electricity surrounded by negative particles of electricity as
shown in the schematic diagram, Fig. 69. The little black circles
inside the large circle represent _positive particles of electricity_
and the little white circles outside of the large circle represent
_negative particles of electricity_, or _electrons_ as East Tennessee Public Communications Corp. Wkop-tv they are
called.
[Illustration: Fig. 69.–Schematic Diagram of an Atom.]
It is the number of positive particles of electricity an atom has that
determines the kind of an element that is formed when enough atoms of
the same kind are joined together to build it up. Thus hydrogen, which
is the lightest known element, has one positive particle for its
nucleus, while uranium, the heaviest element now known, has 92
positive particles. Now before leaving the atom please note that it is
as much smaller than the diagram as the latter is smaller than our
solar system.
What Is Meant by Ionization.–A hydrogen atom is not only lighter but
it is smaller than the atom of any other element while an electron is
more than a thousand times smaller than the atom of which it is a
part. Now as long as all of the electrons remain attached to the
surface of an atom its positive and negative charges are equalized and
it will, therefore, be neither positive nor negative, that is, it will
be perfectly neutral. When, however, one or more of its electrons are
separated from it, and there are several ways by which this can be
done, the atom will show a positive charge and it is then called a
_positive ion_.
In other words a _positive ion_ is an atom that has lost some of its
negative electrons while a _negative ion_ is one that has acquired
some additional negative _electrons_. When a number of electrons are
being constantly given by the atoms of an element, which let us
suppose is a metal, and are being attracted to atoms of another
element, which we will say is also a metal, a flow of electrons takes
place between the two oppositely charged elements and form a current
of negative electricity as represented by the arrows at A in Fig. 70.
[Illustration: Fig. 70.–Action of Two-electrode Vacuum Tube.]
When a stream of electrons is flowing between two metal elements, as a
filament and a plate in a vacuum tube detector, or an amplifier, they
act as _carriers_ for more negative electrons and these are supplied
by a battery as we shall presently explain. It has always been
customary for us to think of a current of electricity as flowing from
the positive pole of a battery to the negative pole of it and hence we
have called this the _direction of the current_. Since the electronic
theory has been evolved it has been shown that the electrons, or
negative charges of electricity, flow from the negative to the
positive pole and that the ionized atoms, which are more positive than
negative, flow in the opposite direction as shown at B.
How Electrons are Separated from Atoms.–The next question that arises
is how to make a metal throw off some of the electrons of the atoms of
which it is formed. There are several ways that this can be done but
in any event each atom must be given a good, hard blow. A simple way
to do this is to heat a metal to incandescence when the atoms will
bombard each other with terrific force and many of the electrons will
be knocked off and thrown out into the surrounding space.
But all, or nearly all, of them will return to the atoms from whence
they came unless a means of some kind is employed to attract them to
the atoms of some other element. This can be done by giving the latter
piece of metal a positive charge. If now these two pieces of metal are
placed in a bulb from which the air has been exhausted and the first
piece of metal is heated to brilliancy while the second piece of metal
is kept positively electrified then a stream of electrons will flow
between them.
Seeing no one that he knew, and his mind weighted anyway with the
menacing mystery of the strange happenings of the night before, he sat
down on a coil of rope, just in the lee of the forward smokestack, to
think the whole matter over for the twentieth time.
He was thus absorbed when something, at first vague and indefinite, then
clearer and clearer until it was unmistakable, began to impress itself
upon his mind. Like the awakening call that comes to a man in a sound
sleep–seemingly as a far-off whisper that gradually gathers volume and
strength until finally the sleeper awakes with a start to find someone
standing directly over him, loudly and insistently calling his name–so
Slim came to a realization of the strange series of sounds that were
being repeated within a few feet of him.
Could it possibly be only the crackling of the steam-pipe that ran along
the smokestack to the whistle–a crackling merely from the pressure
within? For a moment Slim thought an over-wrought imagination was
playing tricks upon him. But he rose hastily and crossed the short
intervening distance.
Clearly and distinctly it came to him then. Someone in another part of
the vessel was rapping desperately upon that pipe! And in the long and
short dashes of the international code that someone was repeating a
single word–”Help! Help! Help!”
In another instant, using the heavy end of his jackknife as a crude
transmitter, Slim was tapping off the reply:
“Who are you–and where?”
“Lieutenant Mackinson,” the message began to come back. “Locked in
closet off engine room. Cant make self heard. Can you help?”
“This is Slim,” the youth rapped back upon the pipe. “Caught your
message on deck. Am coming with help at once.”
And he dashed down the deck toward the captains quarters, almost
bowling over the captains aide as he hurtled into the sanctum of the
ships commander unannounced.
“Well?” the captain demanded sternly. “Why all the haste?”
“Lieutenant Mackinson,” Slim blurted out; “hes locked in a closet down
near the engine room.”
“Locked in a closet!” the captain repeated incredulously. “How do you
know?”
“He gave a telegraphic call for help on the steam-pipe which runs
through there and connects with the whistle,” the lad explained. “I was
on deck and heard it. I talked with him over the pipe.”
“There is no time to lose, then. Come with me.” And the captain himself
hurriedly led the way down through the lower depths of the ship, where
it became hotter and more oppressive with every step they took.
They had taken a route by which they escaped the attention of anyone
else on the ship.
“It should be right about here somewhere,” the captain announced, as
they Kfro 1370 Khz Longview approached a particularly dark passage. For a few steps they felt
their way along, and then stopped to listen.
There was nothing but the dull and constant hum of the engines and the
almost insufferable heat.
“The other side,” said the captain in a lowered voice, as they failed to
find any trace of the imprisoned lieutenant where they were.
They were crossing a short gallery when Slim abruptly signaled a halt.
“I thought I heard something,” he said. “It sounded like another call.”
They stood silent a moment, and then, faint and indistinct, apparently
from somewhere several feet ahead of them, they both heard repeated that
which had made Slim stop. As the letters were tapped off upon the pipe
the lad repeated them for the information of the captain.
“S-M-O-T-H-E-R-I-N-G.”
“Smothering!” echoed the commander of the ship. “Great Scott! I believe
I know now where he is. This way,” and he started down the passageway
toward a narrow stairs leading to a still lower chamber in the vessel.
Three turns–two to the right and one to the left–and the captain
stopped again to listen. Seemingly from within the wall, right at their
elbows, there came a feeble knock. The officer whipped out a pocket
flashlight. They were directly in front of a heavy wooden door. It was
locked.
“Run get a cold chisel or a heavy screwdriver and hammer,” the captain
ordered, and Slim hastened away, to return two minutes later with all
three tools.
“Stand back as far as you can from the door,” said the captain, placing
his lips close to the keyhole. But there was no response from within.
The energy of these oscillations sets up oscillations of the same
frequency in the secondary coil and these high frequency currents
whose voltage is first positive and then negative, surge in the closed
circuit which includes the secondary coil and the variable condenser.
At the same time the alternating positive and negative voltage of the
oscillating currents is impressed on the grid; at each change from +
to - and back again it allows the electrons to strike the plate and
then shuts them off; as the electrons form the conducting path between
the filament and the plate the larger direct current from the B
battery is permitted to flow through the detector tube and the
headphones.
Operation of a Regenerative Vacuum Tube Receiving Set.–By feeding
back the pulsating direct current from the B battery through the
tickler coil it sets up other and stronger oscillations in the
secondary of the tuning coil when these act on the detector tube and
increase its sensitiveness to a remarkable extent. The regenerative,
or _feed back_, action of the receiving circuits used will be easily
understood by referring back to B in Fig. 47.
When the waves set up oscillations in the primary of the tuning coil
the energy of them produces like oscillations in the closed circuit
which includes the secondary coil and the condenser; the alternating
positive and negative voltages of these are impressed on the grid and
these, as we have seen before, cause similar variations of the direct
current from the B battery which acts on the plate and which
flows between the latter and the filament.
This varying direct current, however, is made to flow back through the
third, or tickler coil of the tuning coil and sets up in the secondary
coil and circuits other and larger oscillating currents and these
augment the action of the oscillations produced by the incoming waves.
These extra and larger currents which are the result of the feedback
then act on the grid and cause still larger variations of the current
in the plate voltage and hence of the current of the B battery
that flows through the detector and the headphones. At the same time
the tube keeps on responding Kamiah Valley Inc K09al to the feeble electric oscillations set
up in the circuits by the incoming waves. This regenerative action of
the battery current augments the original oscillations many times and
hence produce sounds in the headphones that are many times greater
than where the vacuum tube detector alone is used.
Now bring a pair of _No. 12_ or _14_ insulated wires from the 110 volt
lighting leads and connect them with a single-throw, double-pole
switch; connect one pole of the switch with one of the posts of the
primary coil of the alternating power transformer and connect the
other post of the latter with one of the posts of your key, and the
other post of this with the other pole of the switch. Now connect the
motor of the rotary spark gap to the power circuit and put a
single-pole, single-throw switch in the motor circuit, all of which is
shown at A in Fig. 22.
[Illustration: (A) Fig. 22.–Top View of Apparatus Layout for Sending
Set No. 2.]
[Illustration: (B) Fig. 22.–Wiring Diagram for Sending Set No. 2.]
Next connect the posts of the secondary coil to the posts of the
rotary or quenched spark gap and connect one post of the latter to one
post of the condenser, the other post of this to the post of the
primary coil of the oscillation transformer, which is the inside coil,
and the clip of the primary coil to the other spark gap post. This
completes the closed oscillation circuit. Finally connect the post of
the secondary coil of the oscillation transformer to the ground and
the clip of it to the wire leading to the aerial when you are ready to
tune the set. A wiring diagram of the connections is shown Hispanic Broadcasters Of Philadelphia, L.l.c. Wwsi at B.
For Direct Current.–Where you have 110 volt direct current you must
connect in an electrolytic interrupter. This interrupter, which is
shown at A and B in Fig. 23, consists of (1) a jar filled with a
solution of 1 part of sulphuric acid and 9 parts of water, (2) a lead
electrode having a large surface fastened to the cover of surface that
sets in a porcelain sleeve and whose end rests on the bottom of the
jar.
[Illustration: Fig. 23.–Using 110 Volt Direct Current with an
Alternating Current Transformer.]
A Two-wire Aerial.–An aerial with two wires will give better results
than a single wire and three wires are better than two, but you must
keep them well apart. To put up a two-wire aerial get (1) enough _No.
16_, or preferably _No. 14_, solid or stranded copper or aluminum
wire, (2) four porcelain insulators, see B in Fig. 5, and (3) two
sticks about 1 inch thick, 3 inches wide and 3 or 4 feet long, for the
_spreaders_, and bore 1/8-inch hole through each end of each one. Now
twist the ends of the wires to the insulators and then cut off four
pieces of wire about 6 feet long and run them through the holes in the
wood spreaders. Finally twist the ends of each pair of short wires to
the free ends of the insulators and then twist the free ends of the
wires together.
For the leading-in wire that goes to the lightning switch take two
lengths of wire and twist one end of each one around the aerial wires
and solder them there. Twist the short wire around the long wire and
solder this joint also when the aerial will look like Fig. 7. Bring
the free end of the leading-in wire down to the middle post of the
lightning switch and fasten it there and connect up the receiver to it
and the ground as described under the caption of _A Single Wire
Aerial_.
[Illustration: Fig. 7.–Two Wire Aerial.]
Connecting in the Ground.–If there is a gas or water system or a
steam-heating plant in your house you can make your ground connection
by clamping a ground clamp to the nearest pipe as has been previously
described. Connect a length of bare or insulated copper wire with it
and bring this up to the table on which you have your receiving set.
If there are no grounded pipes available then you will have to make a
good ground which we shall describe presently and lead the ground wire
from your receiving set out of the window and down to it.
How to Put Up a Good Aerial.–While you can use the cheap aerial
already described for a small spark-coil sending set you should have a
better insulated one for a 1/2 or a 1 kilowatt transformer set. The
cost for the materials for a good aerial is small and when properly
made and well insulated it will give results that are all out of
proportion to the cost of it.
An Inexpensive Good Aerial.–A far better aerial, because it is more
highly insulated, can be made by using _midget insulators_ instead of
the porcelain insulators described under the caption of _A Single Wire
Aerial_ and using a small _electrose leading-in insulator_ instead of
the porcelain bushing. This makes a good sending aerial for small sets
as well as a good receiving aerial.
The Best Aerial that Can Be Made.–To make this aerial get the
following material together: (1) enough _stranded or braided wire_ for
three or four lengths of parallel wires, according to the number you
want to use (2) six or eight _electrose ball insulators_, see B, Fig.
8; (3) two 5-inch or 10-inch _electrose strain insulators_, see C; (4)
six or eight _S-hooks_, see D; one large _withe_ with one eye for
middle of end spreader, see E; (6) two smaller _withes_ with one eye
each for end spreader, see E; (7) two still smaller _withes_, with two
eyes each for the ends of the end spreaders, see E (8) two _thimbles_,
see F, for 1/4-inch wire cable; (9) six or eight _hard rubber tubes_
or _bushings_ as shown at G; and (10) two _end spreaders_, see H; one
_middle spreader_, see I; and one _leading-in spreader_, see J.
[Illustration: (A) Fig. 8–Part of a Good Aerial.]
[Illustration: (B) Fig. 8.–The Spreaders.]
For this aerial any one of a number of kinds of wire can be used and
among these are (a) _stranded copper wire;_ (b) _braided copper wire;_
(c) _stranded silicon bronze wire,_ and (d) _stranded phosphor bronze
wire_. Stranded and braided copper wire is very flexible as it is
formed of seven strands of fine wire twisted or braided together and
it is very good for short and light aerials. Silicon bronze wire is
stronger State Alaska K11qt than copper wire and should be used where aerials are more
than 100 feet long, while phosphor bronze wire is the strongest aerial
wire made and is used for high grade aerials by the commercial
companies and the Government for their high-power stations.
CHAPTER VI
HOW THE TRANSMITTING AND RECEIVING SETS WORK
The easiest way to get a clear conception of how a wireless
transmitter sends out electric waves and how a wireless receptor
receives them is to take each one separately and follow: (1) in the
case of the transmitter, the transformation of the low voltage direct,
or alternating current into high potential alternating currents; then
find out how these charge the condenser, how this is discharged by the
spark gap and sets up high-frequency currents in the oscillation
circuits; then (2) in the case of the receptor, to follow the high
frequency currents that are set up in the aerial wire and learn how
they are transformed into oscillations of lower potential when they
have a larger current strength, how these are converted into
intermittent direct currents by the detector and which then flow into
and operate the telephone receiver.
How Transmitting Set No. 1 Works. The Battery and Spark Coil
Circuit.–When you press down on the knob of the key the silver points
of it make contact and this closes the circuit; the low voltage direct
current from the battery now flows through the primary coil of the
spark coil and this magnetizes the soft iron core. The instant it
becomes magnetic it pulls the spring of the vibrator over to it and
this breaks the circuit; when this takes place the current stops
flowing through the primary coil; this causes the core to lose its
magnetism when the vibrator spring flies back and again makes contact
with the adjusting screw; then the cycle of operations is repeated.
A condenser is connected across the contact points of the vibrator
since this gives a much higher voltage at the ends of the secondary
coil than where the coil is used without it; this is because: (1) the
self-induction of the primary coil makes the pressure of the current
rise and when the contact points close the circuit again it discharges
through the primary coil, and (2) when the break takes place the
current flows into the condenser instead of arcing across the contact
points.
Changing the Primary Spark Coil Current Into Secondary Currents.–Now
every time the vibrator contact points close the primary circuit the
electric current in the primary coil is changed into closed magnetic
lines of force and as these cut through the secondary coil they set up
in it a _momentary current_ in one direction. Then the instant the
vibrator points break apart the primary circuit is opened and the
closed magnetic lines of force contract and as they do so they cut the
turns of wire in the secondary coil in the opposite direction and this
sets up another momentary current in the secondary coil in the other
direction. The result is that the low voltage direct current of the
battery is changed into alternating currents whose frequency is
precisely that of the spring vibrator, but while the frequency of the
currents is low their potential, or voltage, is enormously increased.
What Ratio of Transformation Means.–To make a spark coil step up the
low voltage direct current into high potential alternating current the
primary coil is wound with a couple of layers of thick insulated
copper wire and the secondary is wound with a thousand, more or less,
number of turns with very fine insulated copper wire. If the primary
and secondary coils were wound with the same number of turns of wire
then the pressure, or voltage, of the secondary coil at its terminals
would be the same as that of the current which flowed through the
primary coil. Under these conditions the _ratio of transformation_, as
it is called, would be unity.
The ratio of transformation is directly proportional to the number of
turns of wire on the primary and secondary coils and, since this is
the case, if you wind 10 turns of wire on the primary coil and 1,000
turns of wire on the secondary coil then you will get 100 times as
high a pressure, or voltage, at the terminals of the secondary as that
which you caused to flow through the primary coil, but, naturally, the
current strength, or amperage, will be proportionately decreased.
The Secondary Spark Coil Circuit.–This includes the secondary coil
and the spark gap which are connected together. When the alternating,
but high potential, currents which are developed by the secondary
coil, reach the balls, or _electrodes_, of the spark Kpet 690 Khz In Lamesa gap the latter
are alternately charged positively and negatively.
Now take a given instant when one electrode is charged positively and
the other one is charged negatively, then when they are charged to a
high enough potential the electric strain breaks down the air gap
between them and the two charges rush together as described in the
chapter before this one in connection with the discharge of a
condenser. When the charges rush together they form a current which
burns out the air in the gap and this gives rise to the spark, and as
the heated gap between the two electrodes is a very good conductor the
electric current surges forth and back with high frequency, perhaps a
dozen times, before the air replaces that which has burned out. It is
the inrushing air to fill the vacuum of the gap that makes the
crackling noise which accompanies the discharge of the electric spark.
Though the vacuum tube detector requires two batteries to operate it
and the receiving circuits are somewhat more complicated than where a
crystal detector is used still the former does not have to be
constantly adjusted as does the latter and this is another very great
advantage. Taken all in all the vacuum tube detector is the most
sensitive and the most satisfactory of the detectors that are in use
at the present time.
Not only is the vacuum tube a detector of electric wave signals and
speech and music but it can also be used to _amplify_ them, that is,
to make them stronger and, hence, louder in the telephone receiver and
further its powers of amplification are so great that it will
reproduce them by means of a _loud speaker_, just as a horn amplifies
the sounds of a phonograph reproducer, until they can be heard by a
room or an auditorium full of people. There are two general types of
loud speakers, though both use the principle of the telephone
receiver. The construction of these loud speakers will be fully
described in a later chapter.
Assembled Vacuum Tube Receiving Sets.–You can buy a receiving set
with a vacuum tube detector from the very simplest type, which is
described in this chapter, to those that are provided with
_regenerative circuits_ and _amplifying_ tubes or both, which we shall
describe in later chapters, from dealers in electrical apparatus
generally. While one of these sets costs more than you can assemble a
set for yourself, still, especially in the beginning, it is a good
plan to buy an assembled one for it is fitted with a _panel_ on which
the adjusting knobs of the rheostat, tuning coil and condenser are
mounted and this makes it possible to operate it as soon Wevo Mhz Concord as you get it
home and without the slightest trouble on your part.
You can, however, buy all the various parts separately and mount them
yourself. If you want the receptor simply for receiving then it is a
good scheme to have all of the parts mounted in a box or enclosed
case, but if you want it for experimental purposes then the parts
should be mounted on a base or a panel so that all of the connections
are in sight and accessible.
A Simple Vacuum Tube Receiving Set.–For this set you should use: (1)
a _loose coupled tuning coil,_ (2) a _variable condenser,_ (3) a
_vacuum tube detector,_ (4) an A or _storage battery_ giving 6 volts,
(5) a B or _dry cell battery_ giving 22-1/2 volts, (6) a _rheostat_
for varying the storage battery current, and (7) a pair of 2,000-ohm
_head telephone receivers_. The loose coupled tuning coil, the
variable condenser and the telephone receivers are the same as those
described in Chapter III.
The Vacuum Tube Detector. With Two Electrodes.–A vacuum tube in its
simplest form consists of a glass bulb like an incandescent lamp in
which a _wire filament_ and a _metal plate_ are sealed as shown in
Fig. 37, The air is then pumped out of the tube and a vacuum left or
after it is exhausted it is filled with nitrogen, which cannot burn.
[Illustration: Fig. 37.–Two Electrode Vacuum Tube Detectors.]
When the vacuum tube is used as a detector, the wire filament is
heated red-hot and the metal plate is charged with positive
electricity though it remains cold. The wire filament is formed into a
loop like that of an incandescent lamp and its outside ends are
connected with a 6-volt storage battery, which is called the A
battery; then the + or _positive_ terminal of a 22-1/2 volt dry cell
battery, called the B battery, is connected to the metal plate while
the - or _negative_ terminal of the battery is connected to one of the
terminals of the wire filament. The diagram, Fig. 37, simply shows how
the two electrode vacuum tube, the A or dry battery, and the B or
storage battery are connected up.
Three Electrode Vacuum Tube Detector.–The three electrode vacuum tube
detector shown at A in Fig. 38, is much more sensitive than the two
electrode tube and has, in consequence, all but supplanted it. In this
more recent type of vacuum tube the third electrode, or _grid_, as it
is called, is placed between the wire filament and the metal plate and
this allows the current to be increased or decreased at will to a very
considerable extent.
[Illustration: Fig. 38.–Three Electrode Vacuum Tube Detector and
Battery Connections.]
Now connect one of the posts of the rheostat to one terminal of the
filament Wffc Mhz Ferrum and the other terminal of the filament to the - or _negative_
terminal of the A or storage battery and the + or _positive_ terminal
of the A or storage battery to the other post of the rheostat. Finally
connect the + or positive terminal of the A or storage battery with
the wire that runs from the head phones to the variable condenser, all
of which is shown in the wiring diagram at B in Fig. 41.
Adjusting the Vacuum Tube Detector Receiving Set.–A vacuum tube
detector is tuned exactly in the same way as the _Crystal Detector Set
No. 2_ described in Chapter III, in-so-far as the tuning coil and
variable condenser are concerned. The sensitivity of the vacuum tube
detector receiving set and, hence, the distance over which signals and
other sounds can be heard depends very largely on the sensitivity of
the vacuum tube itself and this in turn depends on: (1) the right
amount of heat developed by the filament, or _filament brilliancy_ as
it is called, (2) the right amount of voltage applied to the plate,
and (3) the extent to which the tube is exhausted where this kind of a
tube is used.
To vary the current flowing from the A or storage battery
through the filament so that it will be heated to the right degree you
adjust the rheostat while you are listening in to the signals or other
sounds. By carefully adjusting the rheostat you can easily find the
point at which it makes the tube the most sensitive. A rheostat is
also useful in that it keeps the filament from burning out when the
current from the battery first flows through it. You can very often
increase the sensitiveness of a vacuum tube after you have used it for
a while by recharging the A or storage battery.
The degree to which a vacuum tube has been exhausted has a very
pronounced effect on its sensitivity. The longer the tube is used the
lower its vacuum gets and generally the less sensitive it becomes.
When this takes place (and you can only guess at it) you can very
often make it more sensitive by warming it over the flame of a candle.
Vacuum tubes having a gas content (in which case they are, of course,
no longer vacuum tubes in the strict sense) make better detectors than
tubes from which the air has been exhausted and which are sealed off
in this evacuated condition because their sensitiveness is not
dependent on the degree of vacuum as in the latter tubes. Moreover, a
tube that is completely exhausted costs more than one that is filled
with gas.