Page:Popular Science Monthly Volume 92.djvu/336

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��Popular Science Monthly

��ance and resistance, but this time in the direc- tion of arrow Y. The process is repeated in- definitely, each charge growing somewhat smaller than the one preceding it, but each producing a half-cycle of alternating current, until the energy of the original charge is all used up in heat or use- ful work. The successive half- cycles in opposite directions unite to produce an alternating current which gradually dies away in strength or amplitude, but whose

����Fig. 32: An arrangement of transmitter depend- ing on condenser discharge in closed circuit

frequency remains constant. In such a con- denser-discharge circuit, the frequency of the current produced depends upon the ef- fective capacity and inductance, and, to a limited extent, upon the resistance. The rate at which the current dies away depends upon the effective values of these same three electrical quantities. The intensity of the current in the first half-cycle of the discharge depends upon the voltage to which the condenser was charged, as well as its size and on the circuit inductance and resistance.

A Sinnple Radio Transmitter

The condenser-discharge method of gen- erating alternating currents is practical for frequencies from a few thousand per second up to millions per second, and is the basis of all radio telegraph transmitters of the spark type. One of the simple ways in which the principle is applied is shown in Fig. 31. A generator G of audio frequency alternating current, of say 500 cycles per second, is con- nected in series with a telegraph key K and the primary coil of a step-up transformer T. The secondary of this transformer, which produces a high voltage (of perhaps 20,000) at the applied frequency of 500 per second, is connected across a spark gap .S which lies between the aerial wires A and the ground E, an inductance coil L being in Beriea in this antenna circuit. Each voltage pulse from the secondary of the transformer puts a charge upon the aerial-ground system, since the wires in the air act toward the surface of the earth much as one plate of a condenser acts toward the other. When approximately the highest point of voltage in each charging pulse is reached, the spark-gap S breaks down and the voltage established between antenna and earth just before the rupture causes a rush of current through the coil L and across the gap S. By choosing suitable values for the inductance capacity and resistance, the discharge can be made to overshoot and to recharge the antenna capacity in the oppo- site direction, just as in the closed circuit of Fig. 4. Thus an alternating current is produced in the antenna-to-earth system. Its frequency is controllable by changing the inductance and capacity of the antenna circuit, and it may be stopped and started by opening and closing the telegraph key. This ofifers one solution to the first problem of radio telegraphy.

Two- Circuit Spark Transmitter

A type of transmitter which is preferred to that of Fig. 31 is shown in Fig. 32. It also depends upon the condenser discharge for generation of radio frequency current in the antenna. Here a condenser C receives the voltage impulses from the transformer, and discharges across the spark-gap S. The radio frequency currents thus generated in the closed circuit S, C, LI induce similar voltages and currents in the antenna circuit A, L3, L2, E by means of the transformer action of the magnetically coupled coils LI and L2. This is perhaps the most effective form of simple spark transmitter which has been used in radio, and forms the basis of by far the greatest number of radio stations in the world. It produces in the aerial wires a controllable radio frequency alternating current, and so satisfies the primary condition of the radio transmitter problem.

This article has necessarily been in the nature of a review of essentials, and could not offer constructive assistance to the student except in so far as it will help to clarify his ideas as to the underlying prin- ciples and relations which exist in the sending apparatus. In the succeeding ar- ticles these principles will be explained in further detail.

(To Be Continued)

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