Radio Broadcast (May-Oct 1922)

202 Radio Broadcast present scheme of radio communication had not come into the field, it seems likely that Trowbridge's scheme would have been universally adopted. If the trailing wire should be one quarter of a mile long, a second ship would be able to detect the presence of the first at a distance of about one half a mile and this would evidently give sufficient warning to prevent collision. ALEXANDER GRAHAM BELL's EXPERIMENTS in 1882 Alexander Graham Bell tried out the scheme of using two charged metal plates immersed in water for communication. Using boats with a submerged plate at the bow and the second plate at the end of a trailing wire one hundred feet long, using interrupted current in one boat and the telephone receiver for the detector in the other, he was able to get signals when the boats were separated about one half a mile. This possible distance will be much less when the boats are in salt water than when in the fresh water of a river, however. In the T. P. S. scheme of the army, two iron stakes are driven into the earth at a separation as great as feasible; a powerful buzzer, with battery and key, is placed in series with the wire which connects these two stakes. If two other stakes are driven into the ground some distance behind the front line trench where the first pair of stakes is driven, and this second pair of stakes is connected by a wire in series with which is a sensitive telephone receiver, the system forms a possible communication link from a position where other types of communication are impossible. HOW MODERN RADIO DIFFERS It is to be noticed that in the schemes of communication so far described the sending and receiving stations each connect two points on the earth's surface and the transmitting and receiving apparatus are connected between these two points; low frequency currents are caused to traverse the earth's surface and a small part of the transmitted current reaches the surface where the receiving points are located. This is true wireless telegraphy, as much so as the type used to-day for radio broadcasting, and the two methods have many points in common. The line connecting the two contact points at the receiving station should be essentially parallel to the similar line at the transmitting station; the transmitted power is sent in all directions in both schemes so that but a very small fraction of the transmitted power is actually received. 1 n the modern radio scheme each station uses two points in a similar manner, but one of them is on the earth's surface and the other is up in the air. The transmitting and receiving antennae should both be vertical, that is, parallel to each other as in the foregoing schemes. The essential difference of the two schemes lies in the frequency of current used in the transmitting antenna, and the factor of height of the two stations. THE IDEA OF MUTUAL INDUCTION A second possible method of wireless communication was opened up when the laws of electro-magnetic induction, discovered independently by Faraday in England and Henry in America, were made known. When a current flows through a coil, a magnetic field is set up in the space surrounding the coil. When the current in the coil is varied, the magnetic field will correspondingly vary, and if another coil is placed in proximity to the first, and so situated in the magnetic field, the changing magnetic field will set up a voltage in the second coil and if this is connected to some detecting device (such as a telephone or galvanometer) any change of current in the first will be recorded in the second. In this method real wireless communication is possible, there being no connection to the earth at either station. The amount of current which can be set up in the second coil by the changing current in the first decreases very rapidly with increasing distance between the two coils, so much so that the scheme is useful over only small distances. Thus if we have two coils say ten feet in diameter, the possible distance of communication would be probably less than two hundred feet. Remarkable as was the discovery of electromagnetic induction it contributed but little directly to the problem of wireless transmission of signals over appreciable distances; it is of course used throughout the transmitting and receiving sets wherever two circuits are coupled together magnetically, but in so far as the actual transmission of the power is concerned it gave but little promise. In 1891, however. Trowbridge suggested an interesting use of this principle, which, had it come about, would have much resembled a modern radio installation. His idea involved the installation of large coils in the rigging of a ship, these coils to be as large as could be carried from the ship's spars. If the current in the coil of one