"After you have your aerial you want to get the rest of the apparatus together. The first thing to do is to get a baseboard which will serve as the bottom of the receiving box. Something like this," and he put his hand on a board about eighteen inches long, twelve inches wide, and about an inch thick. "This is the platform, as it were, on which the different parts of the apparatus are to rest.
"Now since your ear alone can't detect the waves that are coming to and along your aerial, you have to have a sort of electrical ear that will do this for you. Here it is," and he picked up a piece of crystal and a wire of phosphor bronze. "When this wire comes in contact with this bit of crystal the mysterious waves become audible vibrations.
"But this isn't enough. You've got to get in tune with the sending station in order to understand the sounds you hear. When your vibration frequency is the same as that from which the message is sent, you can hear as clearly as though the voice or instrument were in the next room. Now here's a piece of a curtain pole that's about a foot and a half long. You see that I've wound around its entire length, except for about a half inch at either end, a coil of wire. This is called the inductance coil. You will notice that the wire is covered with cotton except for this little strip of wire extending lengthwise where I've scraped the cotton off with sandpaper so as to accommodate the sliding contacts. These sliding contacts can be made from curtain rings with holes punched in them, through which are passed copper rivets. These rivets press against the bare path of the coil and can be moved to and fro until you find the exact point where your set is in tune with the sending station."
CHAPTER VII
IN THE DARK
"Now," continued Dr. Dale, as he glanced round the circle of eager faces, alight with interest in the subject, "we're getting pretty close to the time when one picks up the receiver and begins to listen in.
"But as the electric vibrations, if left alone, would have a good deal of trouble in passing through the telephone receiver, we must have a condenser to help them out. This is very easily made by gluing a piece of tinfoil about one and a half inches square to each side of a sheet of mica. Then you must have two strips of tinfoil, one extending from each side of the mica. If you haven't any mica, a sheet of ordinary writing paper will do, though the mica is better.
"The telephone receiver you will have to buy, as a satisfactory one can't very well be made by an amateur. The receiver ought to have a high resistance to get the best results.
"There," he said, as he laid the telephone receiver on the bench, "those are the essential things you have to have in order to make a set of your own. With these things only, it will of course be a simple set and have a limited range. There are a hundred improvements of one kind or another that you'll learn about as you get more expert, and these can be added from time to time. But the special thing I wanted to prove to you to-day was that it would take only a very small expenditure of money to get this material together. You see how many things I've used that any one of you can find about the house, such as tinfoil, curtain poles, curtain rings, wood for the box, and so on. The wire needed for your tuning coil and your aerial can be obtained for less than a dollar. The detector, including the crystal, can be got for another dollar. An excellent receiver can be bought for two dollars. A few minor things will be needed at perhaps five or ten cents each. Altogether the cost of the set can be brought within five dollars."
This was good news to the boys, many of whom began at once a mental calculation as to the amount of their pocket money, while others began to figure on odd jobs that might bring them in the required amount, in the event that their parents would not supply the money.