"But what I'm getting at is this--that electricity must travel pretty nearly as fast as light--if not faster. So I believe that electricity and light have about the same kind of vibrations, or wave motion."
"Now, then, if they do have--and I admit it's up to me to prove it," went on Tom, earnestly--"why can't I send light-waves over a wire, as well as electrical waves?"
Mr. Swift was silent for a moment. Then he said, slowly:
"Well, Tom, I never heard it argued just that way before. Maybe there's something in your photo telephone after all. But it never has been done. You can't deny that!"
He looked at his son triumphantly. It was not because he wanted to get the better of him in argument, that Mr. Swift held to his own views; but he wanted to bring out the best that was in his offspring. Tom accepted the challenge instantly.
"Yes, Dad, it has been done, in a way!" he said, earnestly. "No one has sent a picture over a telephone wire, as far as I know, but during the recent hydroplane tests at Monte Carlo, photographs taken of some of the events in the morning, and afternoon, were developed in the evening, and transmitted over five hundred miles of wire to Paris, and those same photographs were published in the Paris newspapers the next morning."
"Is that right, Tom?"
"It certainly is. The photographs weren't so very clear, but you could make out what they were. Of course that is a different system than the one I'm thinking of. In that case they took a photograph, and made a copper plate of it, as they would for a half-tone illustration. This gave them a picture with ridges and depressions in copper, little hills and valleys, so to speak, according to whether there were light or dark tints in the picture. The dark places meant that the copper lines stood up higher there than where there were light colors."
"Now, by putting this copper plate on a wooden drum, and revolving this drum, with an electrical needle pressing lightly on the ridges of copper, they got a varying degree of electrical current. Where the needle touched a high place in the copper plate the contact was good, and there was a strong current. When the needle got to a light place in the copper--a depression, so to speak--the contact was not so good, and there was only a weak current."
"At the receiving end of the apparatus there was a sensitized film placed on a similar wooden drum. This was to receive the image that came over the five hundred miles of wire. Now then, as the electrical needle, moving across the copper plate, made electrical contacts of different degrees of strength, it worked a delicate galvanometer on the receiving end. The galvanometer caused a beam of light to vary--to grow brighter or dimmer, according as the electrical current was stronger or weaker. And this light, falling on the sensitive plate, made a picture, just like the one on the copper plate in Monte Carlo."