English: Early telephone designed by US scientist Alexander Graham Bell in January 1877, with the cover off. It was called the "box telephone". It consists of a horseshoe magnet with coils of wire around the pole pieces, located near a sheet iron diaphragm, with a horn mouthpiece. This moving iron driver functioned as both a microphone and earphone. It also had a clapper called the "Thumper", the first signaling device used to signal the other party when a call was coming in. When the caller wanted to initiate a call, he would press the button (left) on the front, which caused the clapper to strike the iron diaphragm. The vibrations would create a large impulse in the coil, which traveled to the receiving party's telephone and generated a loud "thump" in its diaphragm, calling the receiving party to the phone. This was the first commercial telephone. Two or more box telephones in different houses would be connected in a wire circuit with a battery. The first telephone line was installed in April 1877 between Charles Williams electrical shop on Court St., Boston, and his home 3 miles away.

Caption: Box telephone. Cover removed. Made in June, 1877. Equipped with a sheet iron diaphragm and the Watson Hammer Signal or "Thumper". First form of telephone equipped with a signaling or calling device. This instrument was used interchangeably as transmitter and receiver.