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.

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