Though the vacuum tube detector requires two batteries
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.]