“I can understand how the film is made,” responded Bob thoughtfully. “But after it is made, how is the sound reproduced?”
“I was coming to that,” replied the doctor. “The reproduction of the sound from the film is brought about by moving the film in front of an exceedingly delicate electrical device which produces an electromotive force that varies with the amount of light that falls upon it. By an ingenious combination of vacuum tubes, there has been produced an apparatus which responds to variations in the light falling on it with the speed of light itself or with the speed of propagation of wireless waves into space. Therefore, when this film is moved continuously in front of such a device, the device produces an electric current which corresponds very accurately to the original sound wave. This electric current may be used to actuate a telephone or loud speaker.
“When this was told to us last night, I thought that it was the announcer who was talking. But, as a matter of fact, it was the film that was talking. The voice of the announcer had first been recorded on the film and then was sent out with such accuracy that we were all fooled into believing that the announcer himself was speaking to us at first hand.”
“That certainly showed how good it was!” exclaimed Bob. “It’s nothing less than magic! It sometimes seems as though it couldn’t be real—as if radio must be a dream.”
“A dream that has come true,” answered the doctor, as he smilingly said good-by and went on his way.
CHAPTER VI
THRASHING A BULLY
The next morning Bob was on his way to school when on passing the Sterling House, the most prominent hotel in town, he caught sight of the figure of a girl on the porch that looked somewhat familiar to him. He looked again and recognized Nellie Berwick, the orphan girl to whom he and the rest of the Radio Boys had rendered such valuable service when her automobile had run wild and dashed through the window of a store.
At the same moment her eyes fell upon Bob and her face lighted up with pleasure. She waved her hand in greeting, and in a moment Bob had run up the steps and was taking her outstretched hand.
“I’m so glad to see you,” she said, and there was evident sincerity in her voice. “I was just thinking of you before you came in sight.”
“It’s pleasant to be remembered,” replied Bob.