Page:Tom Swift and His Wireless Message.djvu/184

174 certain intervals, making the sounding instrument send out clicks. A short click is called a dot, and a long click a dash. By combinations of dots, dashes, and spaces between the dots and dashes, letters are spelled out. For instance, a dot and a space and a dash, represent the letter 'A' and so on."

"I understand so far," admitted Mrs. Nestor.

"In telegraphing without wires," went on Tom, "the air is used in place of a metallic conductor, with the help of the earth, which in itself is a big magnet, or a battery, as you choose to regard it. The earth helps to establish the connection between places where there are no wires, when we 'ground' certain conductors.

"To send a wireless message a current is generated by a dynamo. The current flows along until it gets to the ends of the sending wires, which we have just strung. Then it leaps off into space, so to speak, until it reaches the receiving wires, wherever they may be erected. That is why any wireless receiving station, within a certain radius, can catch any messages that may be flying through the air—that is unless certain apparatus is tuned, or adjusted, to prevent this.

"Well, once the impulses, or electric currents, are sent out into space, all that is necessary to do is to break, or interrupt them at certain