All the energy expended in the tube appears as heat. It is due to the blows which the electrons strike against the plate when they are drawn across from the filament. These impacts set into more rapid motion the molecules of the plate; and the temperature of the tube rises. There is a limit to the amount the temperature can rise without destroying the tube. For that reason the heat produced inside it must not exceed a certain limit depending upon the design of the tube and the method of cooling it as it is operated. In the Arlington experiments, which I mentioned a moment ago, the tubes were cooled by blowing air on them from fans. We can find the power expended in the plate circuit of a tube by multiplying the number of volts in its battery by the number of amperes which flows. Suppose the battery is 250 volts and the current 0.02 amperes, then the power is 5 watts. The "watt" is the unit for measuring power. Tubes are rated by the number of watts which can be safely expended in them. You might ask, when you buy an audion, what is a safe rating for it. The question will not be an important one, however, unless you are to set up a transmitting set since a detector is usually operated with such small plate-voltage as not to have expended in it an amount of power dangerous to its life. In recent transmitting sets the tubes are used in parallel for the reasons I have just told, but a different method of modulation is used. The generation of the radio-frequency current is by large-powered tubes which are operated with high voltages in their plate circuits. The output of these oscillators is supplied to the antenna. The intensity of the oscillations of the current in these tubes is controlled by changing the voltage applied in their plate circuits. You can see from Fig. 123 that if the plate voltage is changed the strength of the alternating current is changed accordingly. It is the method used in changing the voltage which is particularly interesting. [Illustration: Fig 123] The high voltages which are used in the plate circuits of these high-powered audions are obtained from generators instead of batteries. You remember from Letter 20 that an e. m. f. is induced in a coil when the coil and a magnet are suddenly changed in their positions, one being turned with reference to the other. A generator is a machine for turning a coil so that a magnet is always inducing an e. m. f. in it. It is formed by an armature carrying coils and by strong electromagnets. The machine can be driven by a steam or gas engine, by a water wheel, or by an electric motor. Generators are designed either to give steady streams of electrons, that is for d-c currents, or to act as alternators. [Illustration: Fig 124]