"Can you talk with a ground station when you're flying, say a couple of miles high?" asked Mr. Giddings, examining a transmitter attached to a yoked wire support which his son slipped over his shoulders. "Farther than that. With this particular vacuum tube, which will amplify sounds three or four times over any other I have tried, we expect to talk with ground stations or other aircraft at a distance of three thousand miles. Notice what a simple thing it is, dad," and Bob indicated a little glass bulb which looked a lot like an ordinary incandescent light, but which had a peculiar arrangement of wires and substances inside. "Is the transmitter or receiver made just like the ordinary kind?" asked Mr. Giddings. "Practically the same, dad. The wireless transmitter, like that of the wire telephone, contains a sensitive diaphragm which your voice strikes and sets to vibrating. These vibrations compress and release a capsule of carbon granules which agitate and set in motion an electrical current in two magnets connecting with them. The magnets convey the sound-waves in the form of electrical waves, along wires out to the tip of each wing, where the wires hang down a little way. When a message comes in it is caught by a webbing of antennae wires in our wings." "Then I suppose these sound-waves, in other words the words one speaks, run out of the end of these wires into the atmosphere?" "Exactly, sir," agreed Bob. "That is, the electrical waves are projected into the air and disturb this air in a way to make it pulsate in the same manner as your voice makes the diaphragm pulsate. These waves are then carried through the atmosphere in every direction, and sooner or later reach the antennae wires of some station equipped to receive them. Down these wires they dash, are registered and magnified in the wonderfully delicate vacuum tube, and from it are carried up into the receivers at your ears." "I should think they would be electrical impulses when they reach the receivers," argued Mr. Giddings. "How can a person hear _words_ from electrical discharges?" Bob smiled. "Easy enough, dad," he went on. "You see, this vacuum tube does the business. The electrical current agitates this in unison, and the impulses are immediately converted into words again,--and there you are!" "I acknowledge my understanding now," admitted Mr. Giddings, with a hearty laugh; "but there's just one thing yet I want light on: Where do you get your electrical current? It takes a dynamo to make electricity, else storage batteries. I don't see either." "Come outside here a moment, dad." Bob smiled as he led the little party out of the Sky-Bird's cabin. When they once more stood on the hangar floor, he pointed to a peculiar T-shaped object just beneath the nose of the airplane. This had escaped the gentleman's observation until now. "It looks like a small propeller with a torpedo sticking out from the middle of it," laughed Mr. Giddings.