"Windows," murmured Nancy grasping the idea and translating it into the vernacular.
"Exactly," Bob agreed. Evidently his audience were understanding what he was trying to make clear to them.
"Now we have our high frequency oscillations occurring in the spark discharged from the Leyden jar and jumping the spark gap; nevertheless they would not do us any good were there not some way to use and regulate them. This brings us to the induction coil of which I spoke a second ago."
"It sounds very terrible," smiled Mrs. Crowninshield.
"It isn't, though," answered Bob, returning the smile. "In fact it is a very simple device—nothing more than a dozen or so twists of copper wire reeled about a wooden frame exactly as strands of thread might be wound round a spool. One end of the inductance is connected permanently with the ground and from the other end two movable wires go out, one of which can be connected with the spark gap and the other with the antenna that goes into the air and catches the sound waves. There isn't anything very terrible about that, you see."
"Antenna is what butterflies have," suggested Nancy vaguely.
"Quite right!" assented the wireless man. "Only radio antennæ are not to feel with—at least not in the same way. Nevertheless they do reach out and capture the sound. On all wireless stations you will notice the masts that support them. Sometimes there is one wire, sometimes a group. It is the wires themselves, remember, not the masts, which are the antennæ. Nowadays, however, you will occasionally see an indoor aerial used in connection with small, low-power outfits. It does away with the masts and outside equipment and frequently serves the same purpose quite satisfactorily. But most persons prefer the older method and for long-distance work it has, up to date proved to be indispensable. Now the antenna has both electrical capacity and inductance, and when connected up with the apparatus a wireless operator can at will cause it to disturb the magnetic fields surrounding the earth."
"You didn't say how high these masts had to be, Bob," put in Mr. Crowninshield. "Are they always the same length?"
"Oh, no, indeed, sir," was the prompt response. "Their length varies according to the type of service required of them. I'm glad you asked the question. Sometimes the masts are about two hundred feet high; again they may approximate four hundred and eighteen feet. And sometimes in emergencies you will discover no masts at all, the wires being fastened instead to captive balloons or kites which hold them in place long enough to send or receive hasty messages. This latter method is usually resorted to in wartime or during army or navy maneuvers. There are also compact radio sets to be had that can be carried on mule-back and set up and taken down on a hurried army march. On shipboard the ordinary masts of the vessel serve, of course, to support the antenna."
"Thank you, Bob. That is exactly what I wanted to know," said Mr. Crowninshield.