B, ; 532 MAGNETISM AND ELECTRICITY series of vacuum tubes, each of which ampUfies the current that it receives until there is a sufficient amount to operate a loud-speaker. A Fig. 262. Diagram of a simple radio receiving set. A, antenna ; induction or tuning coils ; C, detector tube ; D, the A battery ; £, the B battery ; F, receiver G, variable condenser ; H, grid leak ; /, ground ; /, fixed condenser. A high-frequency radio wave is known as a carrier wave. Carrier waves may be sent over power lines or telephone wires, and several messages can be transmitted at one time by using carrier waves of different frequencies. Vacuum Tubes May Also Be Used to Generate High-frequency Electromagnetic Waves. In the discussion of the use of the three-electrode vacuum tubes as rectifiers, it was pointed out that such tubes may also be used to produce alternating currents from direct current. Herein lies the value of the three-electrode tube in the transmission of radio waves. If the current from the plate is fed back to the grid, small oscillations in the circuit produced by the tuned circuit will be amplified again and again until an oscillating current of large intensity is produced. Used as oscillators, such vacuum tubes give oscillations which are very powerful. The ordinary broadcasting oscillator may generate from 35,000 to many millions of waves per second; the frequencies used in the United States range from 550,000 to 1,500,000 per second. Sound waves in the microphone vary the current in the oscillator and thus modulate the amplitude of the waves sent out.
RADIO GENERATION AND RECEPTION 533 The older radio sets required batteries for their operation, because of the necessity for a direct current. The direct current is now produced from alternating currents by the use of rectifier tubes, as mentioned in Section 5 of this Unit. A transformer is used to step down the voltage of the current for the filament. The Klystron Will Produce Ultrashort Radio Waves with Considerable Power. Short-wave broadcasting is of advantage because less energy is required in broadcasting, inasmuch as the waves can be focused. As a result of focusing the waves, secrecy is made possible. Short waves also make blind landings of airplanes more practicable. More broadcasting stations are possible because the number of frequencies is greater for a given broadcast range. broadcast in Thirty thousand stations could the range between 9 and 10 centimeters without interfering with each other. This is of especial value in television, inasmuch as no television station can broadcast beyond the horizon. The ordinary triode radio tube is not capable of delivering ultrashort waves, partly because ultrashort waves which complete a cycle in 1/1,000,000,000 of a second do not give enough time for electrons to pass from the negative plate to the positive plate between changes in plate voltage. W. W. Hansen developed the Klystron, which consists of two rumbatrons. The principle of the rumbatron is that pulsations are produced by changing the velocity of a stream of electrons, rather than by shutting off and on a current of electrons. The Klystron produces ultrashort waves which are as ef^cient as long radio waves and thus makes possible important advances in short-wave radio broadcasting. Already ultrashort waves have been employed successfully to relay television programs, thus making possible television network systems. In 1940 the Klystron tube was used to demonstrate the transmission of power in which members of an audience held flashlight bulbs that were lighted by short waves transmitted from a Klystron tube located on the platform of an auditorium. It is interesting to note that these short waves, unlike ordinary radio waves, do not penetrate nonmetallic materials such as wood. Loud-speakers and Microphones Added Much to Radio Broadcasting and Reception. Less distortion and greater volume of sound are obtained with the use of loud-speakers than were obtained with telephone receivers. In