The Thermionic Valve in Wireless Telegraphy and Telephony¹

Abstract

THE thermionic valve is an invention which has vastly increased the powers and range of wireless telegraphy. Like many other inventions, the telephone, for instance, it is simple in its essential construction. It consists of a little electric lamp comprising a glass bulb, very highly exhausted of its air, containing a filament of carbon, or better tungsten, which can be rendered incandescent by an electric current. Within the bulb and around the filament are fixed certain metal plates or cylinders, and, it may be, spirals of wire or metal networks called the grid. To explain its origin in its simplest form I shall have to take you back in thought to the days when the physical effects taking place in incandescent electric lamps were first beginning to be considered carefully. In 1883 Mr. Edison for some purpose placed in the glass bulb of one of his carbon filament lamps a metal plate which was carried on a platinum wire sealed through the glass. When the filament was rendered incandescent by a current from a battery, he found that if the plate was connected by a wire, external to the lamp, with the positive terminal of the filament, a small electric current flowed through it, but if connected to the negative terminal no current, or at most a very feeble current, flowed. This new and interesting effect became known as the “Edison effect” in glow lamps, but Mr. Edison gave no explanation of it, and made no practical application of it.