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SEEING SOUND. You can also demonstrate the vibrations that sound creates by converting your pick-up to a recorder. Do this by replacing the needle with the ballpoint cartridge. Then cut a round piece of smooth cardboard the size of your phonograph turntable. Punch a hole in the middle, and place it on the turntable. Start the machine. As you hold the pen gently on the card- board, make various sounds into the can. Note the wavy line caused by the vibrations of the dia- phragm as the sounds from your voice strike it.

THE TELEGRAPH RELAY

     You’ve heard of a relay, haven’t you? It’s an electromechanical device that allows a weak force to control a much stronger force. For example, a relay allows a switch in a safe low-voltage circuit to open or close a higher-voltage circuit.

     In the telegraph system that Edison operated and helped develop, the current flowing in the line between two distance stations was too weak to operate the sounder. The sounder is what made the coded clicks that the operator would translate into a message. So to activate the sounder, a relay was used.

     Here’s how it worked: The telegraph line was connected to a coil in the relay having many turns of wire around an iron core. When current flowed through the coil, meaning that a message was coming in, the coil became an electromagnet. The resulting magnetism caused two contacts in the relay to close. These contacts were part of a sepa- rate circuit that included the sounder and its own power supply. How the click intervals varied indi- cated what the message was.

     Building and testing an electromechanical relay will give you a clearer picture of this operation. In your setup, imagine the telegraph key (really a kind of on-off switch) to be miles away and the relay located at your end of the line. A light bulb will serve as the sounder.