Page:Popular Science Monthly Volume 91.djvu/461

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Popular Science Monthly

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���The aerial is made of copper tubing

��tells him that the shots from his artillery are not reaching the target.

"D 300, L 100," he signals. That means, "Lower the range 300 yards and come 100 yards to the left."

A few more seconds, and there is another puff of white smoke.

Again it is short of JJPL/enai coil

the target.

"U 100, R 50," he signals; mean- ing, "Raise the range a hundred yards and shift it fifty to the right."

Again he waits, and this time he has his reward. Now the puff comes from exactly the right spot.

"H," he signals back, repeating the letter several times. It means, "Hit."

The artillery have got the range.

Sometimes they get it more easily than this. But often they take much longer. And the longer they take, of course, the more perilous for him. For meantime, naturally, he himself has become a target. Shells from the anti-aircraft guns have been exploding to right and left of him, but he manages to soar away from these. Then something worse looms up. The enemy airforce is preparing to meet him. They are leaving the ground. He knows they will soon be on him. Unless he hastily disappears from the scene there is soon in progress one of those great duels in the air — but that is another story.

��Down with the Old -Time School Bell! Supplant It With Wireless

ONE of the technical high schools in a large city has worked a new variation on that old-time institution — the public school bell. Time was when brassy- sounding bells used to adorn every school- house steeple in the land. In the larger cities these have retired in favor of a system of electric gongs, one in every room. This system is useful for calling classes during a day as well as for gathering the whole flock of pupils together at nine in the morning — this last being about the sole function the earlier steeple bells could perform. But this school in question goes a step farther than even the electric gongs, connected as the gongs are with networks of wires, annunciators, time-clocks and the like. It is done here by wireless.

��The change came about through the fact that the school long ago became too big for the original building, even though it was large and well equipped. Since then surplus classes have been held in such buildings in the neighborhood as could be secured. How to call and adjourn simul- taneously the successive classes throughout the day in all these buildings therefore became a problem. The principal sug- gested that it be done by wireless, and the school's class in radio-telegraphy com- pleted the system, the essentials of which are shown in the diagram.

The sending end of the system is unusual only in that a small rotary converter is used to convert direct current — the only supply available — to alternating, for the trans- former. Had alternating current been available this could have, of course, been hooked on at the same place as that of the converter, and the system would have worked as well. The receiving circuit uses a coherer made of metal filings in a small glass tube in the usual way. Interposed between the coherer and the bells, which do the calling of classes, is a pony relay of the kind often used on ordinary railroad telegraph circuits. The object in using a relay is that its high resistance takes less current than the bells would if connected directly to the coherer. When heavy currents go through a coherer they cause the filings to stick together permanently, or nearly so, rendering them hard or prac-

���Instruments in use by a technical high school for calling the classes by wireless

tically impossible to decohere. The co- herer, as is evident from the diagram, is placed in close proximity to the clapper on one of the bells. Thus the clapper automatically decoheres the coherer, doing away with any special apparatus for that purpose.

A one-kilowatt transformer sends out a wave sufficiently powerful to operate co-

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