426. The Action of the Wireless Telephone.—In the wireless telephone we have a continuous stream of electric waves of high frequency. (See Fig. 425A.) This stream of electric waves corresponds to the current that flows through the transmitter in the wire telephone. These waves are of such high frequency that even though we had a receiver diaphragm vibrating in step with the waves, we could not hear the sound because the human ear cannot hear a sound which consists of more than about 40,000 vibrations per second. The sound waves act upon this stream of waves very much, as in the wire telephone, the transmitter acts to modify the line current. The impulses caused by the voice are much slower than the electric waves first mentioned and these slower impulses are reproduced in the receiver. Not only are these slower impulses reproduced but they are amplified, that is, produced with greater energy than the impulses impressed on the stream of waves. Fig. 425A represents as nearly as is possible in a diagram the continuous stream of electric waves. Fig. 426B, represents the impulses produced by the sound alone, and Fig. 426C, shows how these voice impulses are impressed on the stream of waves.
Fig. 425.—A, unmodified high frequency waves; B, waves of voice frequency; C, high frequency waves modified by waves of voice frequency.
Fig. 426.—Vacuum tube, transmitting type. (Western Electric Co.)
Fig. 427.—Vacuum tube, receiving type. (Western Electric Co.)
Fig. 428.—Diagram of wireless telephone transmitting set.
427. The Vacuum Tube or Audion.—The device by which all of this is accomplished is the vacuum tube. (See Fig. 426.) This tube contains three electrodes. First, a filament (F, in Fig. 428) which is heated by a current from a battery (B1, Fig. 428) and because it is heated, sends out a stream of electrons. Second, the plate which forms the anode of the circuit from battery, B2. This plate receives the electrons which are thrown off by the heated filament, hence a current flows through the circuit of B2; the discharge through the tube depending on the e.m.f. between the filament and the plate. Third, a grid is placed between the filament and the plate and is connected to the secondary of the induction coil, the primary of which is connected to the transmitter. When the transmitter diaphragm is vibrating, the e.m.f. induced in the secondary of the induction coil causes a variation in the potential of the grid. This means a variation in the electric field between the filament and the plate. (See Fig. 428.) The changing electric field causes a variation in the discharge of electrons through the tube; the variation corresponds to the vibrations of the transmitter diaphragm. This produces a surging current of the frequency of the sound waves in the primary of the transformer (T, Fig. 428). The secondary of this transformer is connected to the antennæ (A) and the earth (E). By means of the transformer, rapid surgings are set up in the antennæ and these surgings produce a continuous stream of electromagnetic waves which goes out in space. (Like Fig. 426C.) These electromagnetic waves produce oscillations in the antennæ of a receiving station. The antennæ transmit the impulses to a tube (Fig. 427) which acts as a detector, and makes possible the reproduction of the sound by an ordinary telephone receiver.
Fig. 429.—View of wireless telephone set.