"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.
With a few deft movements the doctor attached the various parts of the apparatus to their proper places on the baseboard. There was not time that day to put up the aerial, but he gave them practical illustrations of how to use the detector by pressing the point of the wire firmly against the crystal, how to slide the rings back and forth until they found the point of greatest loudness and clearness, and all other points essential to using the set successfully. Not all the boys caught on to all that was involved, but to the majority it was made reasonably clear. To Bob and Joe, who had followed every point of the demonstration with the keenest attention, the operation of the receiving set was made as clear as crystal, and they had no doubt of their ability to construct a set for themselves. Herb's attention had wandered somewhat, because in the back of his mind there still lurked the idea of buying a set ready made. Jimmy had been somewhat distracted by looking about in various parts of the barn to see if he could detect the presence of any "eats," and his ideas were somewhat hazy in consequence.
