528 MAGNETISM AND ELECTRICITY well's theory, waves would be set up which would travel out in all directions from the vibrating electron. A luminous body is considered to have within it countless vibrating electrons passing from one energy level to another, which cause the electromagnetic waves that we call light. A heated, nonluminous body would also contain vibrating electrons, but the amplitude of the vibration would differ from that in a luminous body, and the wave length of the resulting electromagnetic waves would therefore be different. In this case such waves would be called infrared, or heat, waves. Hertz Produced Electromagnetic Waves by an Oscillating Electric Charge. The German physicist, H. IlelmhoUz, suggested to one of his brilliant students, Ileinrich Hertz, that he attempt to obtain experimental proof of Maxwell's theory. Hertz accepted the challenge. His first problem was to determine whether or not electromagnetic waves would travel through space without a conductor. While working on this problem, he observed a tiny discharge between the extremities of Fig. 258. Apparatus used by Hertz to demonstrate that electric oscillations produce electromagnetic waves. a flat coil when a nearby Leyden jar was producing an electric discharge between two knoblike terminals. This gave him the clue to the knowledge he was seeking. He produced an oscillating charge by connecting the terminals of an induction coil, A and 0, to two metal plates, B and B, which acted as a condenser. By the simple process of charging and discharging this condenser, he produced electric oscillations. That these oscillations really produced electromagnetic waves he then proceeded to prove by use of a loop of wire, CD, with a small spark gap. He found that when it was properly oriented sparks jumped across this gap in the loop of wire at the same instant that they jumped between the terminals of the induction coil across the room. The confirmation of Maxwell's electromagnetic theory of radiation was one of the greatest advances ever made in the realm of physical science, because it not only presented a principle by which knowledge of radiant energy could be coordinated, but it paved the way for wireless telegraphy and the radio.
; RADIO GENERATION AND RECEPTION 529 Marconi Invented the Wireless Telegraph. Hertz considered the possibility of using these "wireless" waves for communication but he could not devise a method to detect the wave ^ at a distance. Discoveries of other men furnished the basis for wireless wave reception, but it remained ^ first for Marconi to work out its use in actual communication. Guglielmo Marconi (1874- ) improved the sending apparatus so as to give waves of greater intensity. The plates of the condenser had to be enlarged, and Marconi conceived the idea of using a system of wires (or antennae) supported high in the air as one plate of the condenser and tc ^B "r iiiiiiiiiit the ground as the other plate. A high-frequency induction coil operated by a battery produced Fig. 259. Marconi's sending apparatus : A, antenna ; B, induction coil the current, and the signals were C, spark gap ; D, key ; E, battery produced by making and breaking the current with a telegraph G, ground. key. The spark was produced between two wires, one leading from the ground and the other leading from the antennae. The spark produced the electromagnetic waves (now called Hertzian or radio waves) , which were detected by the receiving set. The oscillations in the receiving circuit are very feeble, but their effective reception has been greatly increased by the use of antennae. Marconi used a coherer tube as a detector. The principle of the coherer had been worked out by other investigators, but they did not realize its possibilities. The principle is simple; a tube of nonconducting material, such as glass, is filled with metal particles. When the particles are subjected to electromagnetic waves, they cohere and thus increase the ability of the particles to conduct a current. Marconi used an electric tapper to cause the particles to decohere after each impulse. A current was thus caused to operate a telegraphic sounder so that Morse signals could be received. An early form of detector consisted of a crystal of galena in contact with a piece of copper wire. Such a crystal permitted the current to pass more readily in one direction than in the opposite direction and thus converted the oscillations into impulses in one direction which could be detected by a telephone receiver. Perhaps some of the E