. Discovery. Science. 208 DISCOVERY phone receiver, which was joined up to the ends of the coil. In the electrolytic type of detectors, as in coherers, the oscillatory currents reduced the resistance of the circuit in such a way as to allow the passage of a current suitable for working a telephone receiver. The detector consisted of two dissimilar metals, such as platinum and lead, immersed in an acid solution, the platinum having only a small point, the lead a large surface, in contact with the liquid. Wlien at rest, a non-con- ducting film formed on the point, causing the circuit through the
. Discovery. Science. 208 DISCOVERY phone receiver, which was joined up to the ends of the coil. In the electrolytic type of detectors, as in coherers, the oscillatory currents reduced the resistance of the circuit in such a way as to allow the passage of a current suitable for working a telephone receiver. The detector consisted of two dissimilar metals, such as platinum and lead, immersed in an acid solution, the platinum having only a small point, the lead a large surface, in contact with the liquid. Wlien at rest, a non-con- ducting film formed on the point, causing the circuit through the liquid from one metal to the other to have a high resistance. Oscillatory currents disturbed this film, and the resistance decreased so that the current for working the telephone could flow through the circuit. In 1904 Professor J. A. Fleming in this country invented a detector which he called the Oscillation Valve. This consisted of an incandescent lamp with the addition of a metal plate, or cylinder, inside the bulb, connected to a wire sealed through the glass. When the lamp is glowing, the space between the filament and the plate is electrically conductive, but in one direction only—that is to say, it offers a very great resistance to a current in one direction, but very little resistance to a current in the opposite direction. If, therefore, an oscillatory current is applied, it becomes, for all practical purposes, a unidirectional pulsating current which can be used for working a tele- phone receiver. The device acts, in fact, as a valve for the oscillations, allowing current to flow in one direction but not in the other, and was therefore called an oscillation-valve, the resulting current being called a rectified current. Soon after the introduction of the Fleming Valve, another form of receiver, called the Crystal Detector, was introduced. The first practical form was used by General Dunwoody in the United States, and consisted of a crystal of Carborundum in contact
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