Popular Science Monthly 



445 



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- 



