Before inventing the telephone, Bell was a professor of vocal physiology and elocution at Boston University, where experiments with a real human ear demonstrated the sound vibrations acting on the middle ear bones. From this observation, he went on to invent a voice recording device called a "phonautograph" that traced the pattern of sound waves from a person's voice on glass. A simple experiment in "seeing sound" lets kids imitate Bell's quest to understand sound. Remove both ends of a tin can, stretch a balloon tightly over one end and secure with rubber bands. Glue a small mirror in the center and set the can facing a white wall. Shine a light at a 45-degree angle at the mirror so that a light spot reflects onto the wall. Ask a partner to shout into the open end of the can and watch the light move with the sound vibrations. Science Buddies offers another option to build a wire recording device.
Bell's dream was to create a multi-message telegraph capable of sending up to 40 messages at the same time and even send a human voice over the telegraph wire. He experimented with fluctuating the resistance in an electrical circuit to match the sound frequency so as to hear a message from an electromagnetic relay. He tested his theory with a tuning fork, a battery, a bowl of water, conductive acid and a relay and succeeded in passing the tuning fork tone through the relay. Kids can replicate his experiment by connecting an old telephone receiver, headphones or a loudspeaker to a 6-volt battery. Run a second wire from the relay device to a tuning fork and connect the second battery terminal to the handle of a metal measuring cup filled with vinegar. Hit the tuning fork to start it vibrating. Lower it horizontally into the vinegar and listen for the sound coming through the relay. Insert the tuning fork vertically into the liquid and compare results.
A liquid transmitter was the final step before Bell unexpectedly succeeded in his quest to transmit a human voice over a wire with his famous words, "Mr. Watson come here. I want to see you." Building off his success in the tuning fork experiment, Bell replaced the tuning fork with a wire. For the students, screw a piece of wax paper between two adaptor connectors until just taut. Bolt a copper cup to a wooden board and fill with vinegar. Glue a piece of 24-gauge bare bus bar wire to the wax paper membrane. Make it just long enough to allow the end of the wire to make contact with the vinegar. Connect a relay or recording device to the vibrating wire and one battery terminal. Connect the second battery terminal to the bolt under the copper cup. Place a cardboard or paper cone over the vibrating membrane. Ask a partner listening on the receiver, or record the sound transmission and speak into the cone. The Antique Telephone History website has detailed instructions on how to build one type of liquid transmitter design.
A string or wire telephone is perhaps the most common and simplest telephone science experiment for kids. Connect two paper cups or tin cans with a long piece of wire or string and kids can try sending messages through the line by speaking or shouting into their receivers. Use different types of cans and cups, such as metal, glass, paper or plastic. Compare results with string, twine, yarn and different wire types to find out which is the best sound transmitter.