The rectifiers utilize the property of a vacuum tube (which permits electrical
current through it in but one direction) to change alternating current to direct current suitable for battery charging. Before going into the description of the Tungar
or Rectigon battery chargers used with Photophone equipment it is better to understand something about the action of the rectifier tubes used in them. This
knowledge is particularly important in order to understand the operation of vacuum
tubes in the amplifiers which will be described in a later chapter.
While experimenting with incandescent lamps, Edison discovered in 1876, what
has since been known as the "Edison effect." He found that, when a source of AC
was so connected as to place an alternating voltage between the heated filament of a
lamp and a metal plate within the bulb separated from (not touching) the filament, a current would flow from the plate to the filament but not from the filament
to the plate.
This effect was subsequently explained as follows:-When any substance is
heated to incandescence in a vacuum, it throws off into the space surrounding it
vast quantities of invisibly small particles of negative electricity. These particles
of negative electricity, as stated in Chapter II, are called electrons. Some substances throw off, or emit, electrons much more readily than others, and the hotter
the substance the greater is the number of electrons emitted. The reason for this
is that all matter is largely composed of these particles of negative electricity,
which are always in rapid and violent motion. The increase of temperature increases the speed and violence of their motion. There is always an attractive force
between these electrons and the substance, but when they attain a high speed some
of them overcome the attractive force and leave the surface of the substance, only
to return again unless some outside force carries them away. If a plate is placed
within the tube and kept at a positive potential (voltage) with respect to the heated
substance, some of the electrons will be attracted to it and a current will be established. But if the plate is negative with respect to the heated substance, the electrons will be forced back into the surface from which they were emitted and no current will flow. Therefore the tube acts as an electric valve, permitting current to
flow in one direction but not in the other.
It will be noticed that in the above discussion it was stated that the electrons
flow from a negative, heated substance to a positive plate. This direction of
flow is contrary to the usual conception of the direction of flow of electricity, which is
considered to be from positive to negative. The reason for this is that before the
discovery of electrons, experimenters decided to consider that current flowed from
positive to negative as a sort of arbitrary rule. This rule has continued in use even
though later experiments seemed to prove the contrary to be true. Therefore, current is always considered to flow from positive to negative, although the electrons
actually travel in the opposite direction. The important thing to remember about
vacuum tubes containing a heated filament and a plate is that the current flows in
only one direction between the plate and the filament.