=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 (_B_{1}_, 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, _B_{2}_. This plate receives the electrons which are thrown off by the heated filament, hence a current flows through the circuit of _B_{2}_; 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. 426_C_.) 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. [Illustration: FIG. 429.--View of wireless telephone set.] The _vacuum tube_ in the transmitting circuit also _amplifies_ the impulses, that is, the energy of the waves given out is greater than that of the impulses which produce them, the additional energy being derived from the battery sending current through the plate and filament. In operation, the filament and the plate are connected to a battery with a _condenser_ (_VC_) and an _inductance coil_ (_I_) in the circuit, as shown in Fig. 428. Photograph of a complete modern wireless telephone set is shown in Fig. 429. ALTERNATING CURRENTS