These oscillations pass through the primary of the oscillation transformer, inducing in the secondary, electric oscillations which surge back and forth through the antennæ, or aerial wires, A. These oscillations set up the "wireless waves." The production of these waves is explained as follows: An electric current in a wire sets up a magnetic field spreading out about the conductor; when the current stops the field returns to the conductor and disappears. The oscillations in the antennæ, however, have such a high frequency, of the order of a million a second, that when one surge of electricity sets up a magnetic field, the reverse surge immediately following sets up an opposite magnetic field before the first field can return to the wire. Under these conditions a succession of oppositely directed magnetic fields are produced which move out from the antennæ with the speed of light and induce electric oscillations in any conductors cut by them.
Fig. 416.—Diagram of a commercial wireless telegraph apparatus.
While the electric waves are radiated in all directions from the aerial, the length of the waves set up is approximately four times the combined length of the aerial wires and the "lead in" connection to the oscillation transformer.
The electric waves induce effective electrical oscillations in the aerial of the receiving station, even at distances of hundreds of miles, provided the receiving transformer, RT, is "tuned" in resonance with the transmitting apparatus by adjustments of the variable condenser, VC, and the loading coil, L. The detector of these oscillations in the receiving transformer is simply a crystal of silicon or carborundum, D, in series with two telephone receivers, Ph. The crystal detector permits the electric oscillations to pass through it in one direction only. If the crystal did not possess this property, the telephone could not be used as a receiver as it cannot respond to high frequency oscillations. While one spark passes at SG, an intermittent current passes through the receiver in one direction. Since some 300 to 1200 sparks pass each second at SG while the key, K, is closed, the operator at Ph hears a musical note of this frequency as long as K is depressed. Short and long tones then correspond to the dots and dashes of ordinary telegraphy. In order to maintain a uniform tone a rotary spark gap, as shown, is often used. This insures a tone of fixed pitch by making uniform the rate of producing sparks.
The Continental instead of the Morse code of signals is generally employed in wireless telegraphy, since the former employs only dots and dashes. The latter code employs, in addition to dots and dashes, spaces which have sometimes caused confusion in receiving wireless messages. The United States government has adopted the regulations of the International Radio Congress which directs that commercial companies shall use wave lengths between 300 and 600 or above 1600 meters. Amateurs may use wave lengths less than 200 meters and no others, while the government reserves the right to wave lengths of 600 to 1600 meters. See p. 459 for Continental telegraph code.
418. Discharges in Rarefied Air.—Fig. 417 represents a glass tube 60 or more centimeters long, attached to an air pump. Connect the ends of the tube to the terminals of a static machine or of an induction coil, a-b. At first no sparks will pass between a and f, because of the high resistance of the air in the tube. Upon exhausting the air in the tube, however, the discharge begins to pass through it instead of between a and b. This shows that an electrical discharge will pass more readily through a partial vacuum than through air at ordinary pressure. As the air becomes more and more exhausted, the character of the discharge changes. At first it is a faint spark, gradually changing until it becomes a glow extending from one terminal to the other and nearly filling the tube.
Fig. 417.—An Aurora tube.