Page:Popular Science Monthly Volume 92.djvu/975

 ropular Science Monthly

��a number of the units or pairs are con- nected in series. This construction has received the name of "quenched gap," since when it was first used it was pre- sumed to have an especially effective quenching or extinguishing action upon the spark passing across it. The quench- ing action is now generally held to be more dependent upon the correlation of circuit adjustments than on the gap structure itself, however, as will be explained later, and the gap is merely one form of device which is capable of giving extremely uniform operation. This regularity of operation depends largely upon the fact that the current through the gap is kept small, and the surfaces are kept clean and parallel, so that successive discharges take place from different portions of the discharge plates. Thus extreme heating of any one point is prevented, and the gap may be relied upon to discharge at a quite definite potential time after time.

The basic operation is exactly as in all the other spark gaps considered. The construction shown in Fig. 44 involves plates having cooling flanges, separating feet and the silver sparking surfaces in- dicated respectively by F, G and H, which are placed face to face (spaced accurately by the insulating gaskets J) in pairs as shown. The entire group of from six to thirty or forty units is clamped together in a special form of rack.

This quenched gap is connected into the normal circuit of Fig. 41, replacing the rotary gap there shown. For best operation it requires a circuit adjustment som.ewhat different from that used with the rotary gap, since with "quenching" operation the endeavor is to transfer the energy of the radio frequency oscil- lating currents across the transformer with primary Li, into the antenna cir- cuit, in the shortest possible time. Under these conditions, the gap discharges so regularly that a pure musical signal tone is heard at the receiver. The tech- nical differences between so-called "quench- ing" and "non-quenching" operation may be understood from a study of such authoritative treatises as Ze- meck's "Wireless Telegraphy"; the princi- ples given in the foregoing are sufficient to bear in mind during the first survey of the action. The war-time student requires only such a grasp of the general subject

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of radio as will permit him to get into practical field work at the earliest moment.

Production of Radio Waves

The oscillating currents of the con- denser circuit (Fig. 41) are transferred into the antenna and ground circuit by the transformer action of coil L, upon its secondary. Similar radio frequency cur-

rents are consequently generated, in groups, in the aerial wires which stretch up- ward to the mast top. These anten- na currents result in the production of radiant ether waves of the same radio frequency. The wireless waves pass outward over the surface of the earth, in all directions, at the speed of 186,000 miles per second; some small portion of the radiated wave energy reaches the receiver to which the messages are sent. The closing articles of this series will take up the interception of the radio waves at the receiving station.

���f!q.44 Separating feet and sparking surfaces

��Mind Reading by Wireless — Try It on Wise Friends

THE mind reader bounds up every few months. The feats he can perform are little short of marvellous. William Dubilier of New York has given the fol- lowing details to the Popular Science Monthly of how many of these mind readers work. Any wireless enthusiast can set up equivalent apparatus and obtain equally good results.

Dress yourself or a friend up as the mind reader. Oriental costumes are all the style in the mind-reading profession, and in this case are especially necessary because a Turkish or Indian turban makes an excellent headpiece by which the mind reader can conceal a pair of tele- phone receivers clamped to his ears. Around his body just beneath the shoul- ders are wound some 100 turns of No.

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