First there was a vacuum tube oscillator which generated a small current of the desired frequency. Then there was a telephone transmitter which made variations in a direct-current flowing through the primary of a transformer. The e. m. f. from the secondary of this transformer and the e. m. f. from the radio-frequency oscillator were both impressed upon the grid of an audion which acted as a modulator. The output of this audion was a radio-frequency current modulated by the voice. The output was amplified by a two-stage audion amplifier and supplied through a coupling coil to the large antenna of the U. S. Navy Station at Arlington. Fig. 121 shows the system.
The audion amplifiers each consisted of a number of tubes operating in parallel. When tubes are operated in parallel they are connected as shown in Fig. 122 so that the same e. m. f. is impressed on all the grids and the same plate-battery voltage on all the plates. As the grids vary in voltage there is a corresponding variation of current in the plate circuit of each tube. The total change of the current 234in the plate-battery circuit is, then, the sum of the changes in all the plate-filament circuits of the tubes. This scheme of connections gives a result equivalent to that of a single tube with a correspondingly larger plate and filament.
Parallel connection is necessary because a single tube would be overheated in delivering to the antenna the desired amount of power. You remember that when the audion is operated as an amplifier the resistance to which it supplies current is made equal to its own internal resistance of RP. That means that there is in the plate circuit just as much resistance inside the tube as outside. Hence there is the same amount of work done each second in forcing the current through the tube as through the antenna circuit, if that is what the tube supplies. “Work per second” is power; the plate battery is spending energy in the tube at the same rate as it is supplying it to the antenna where it is useful for radiation.
Pl. XI.–Broadcasting Equipment, Developed by the American Telephone and Telegraph Company and the Western Electric Company.
235All 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 236method 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.