The same things are true of the wireless telegraph and telephone. Instead of an electric current, let us say "a stream of electric waves." Then we may say of the wireless everything that we have said of the wire telegraph and telephone. In sending a message by wireless telegraph the stream of electric waves flows when the key is pressed and stops when the key is released. We have an interrupted stream of electric waves. But an interrupted stream of waves cannot be used for a wireless telephone any more than an interrupted current can be used for a wire-telephone. There must be a constantly flowing stream of electric waves, and these waves must be changed in strength and form by the sound of the voice. Fig. 111 shows a wireless-telephone receiver in which light is used to carry the message. The light acts on the receiver in such a way as to reproduce the sound. [Illustration: FIG. 111--RECEIVER OF BELL'S PHOTOPHONE An early idea in wireless telephony.] In the wireless-telegraph receiver the interrupted stream of electric waves makes and breaks the circuit of an electric battery. The wireless-telephone receiver must not make and break a circuit, but it must be sensitive to all the changes in the electric waves. One such receiver is the audion, which we shall now describe. The audion was invented by Dr. Lee de Forest. De Forest had taken the degree of Doctor of Philosophy at Yale University, having written his thesis for that degree on the subject of electric waves. He then entered the employ of the Western Electric Company in Chicago, and while in this position worked at night in his room on experiments with electric waves. Here he found that a gas flame is sensitive to electric waves (Fig. 112). If a gas flame is made part of the circuit of an electric battery, which includes also an induction-coil connected to a telephone receiver, then when a stream of electric waves comes along there is a click in the receiver. The waves change the resistance of the flame, and so change the strength of the current. The flame is a simple audion. It is the heated gas in the flame that responds to the electric waves. [Illustration: FIG. 112--A GAS FLAME IS SENSITIVE TO ELECTRIC WAVES] If instead of a gas flame an incandescent-light bulb is used having a metal filament, and on either side of the filament a small strip of platinum, a more sensitive receiver is obtained. This is the audion, which is the distinguishing feature of the De Forest wireless telegraph and wireless telephone. The metal filament is made white hot by the current from a storage battery. The vacuum in the bulb is about the same as that of the ordinary incandescent electric light. A very small quantity of gas is therefore left in the bulb. The electrified particles of gas respond more freely to electric waves in this bulb than in the gas flame.