English: Early experimental telephone transmitter (microphone) and receiver (earphone) invented by US scientist Alexander Graham Bell in 1876. The wooden bases on which the devices are normally mounted have been removed. These may have been the prototypes exhibited at the US Centennial Exposition in Philadelphia in summer 1876 which attracted Bell nationwide attention. Both were constructed similarly of a parchment diaphragm stretched over the end of an acoustic horn, with a piece of soft iron cemented to the center, and an enectromagnet suspended near it. The coils of the two instruments were connected in a circuit with a battery. The current of the battery created a magnetic field in each coil. When a person spoke into the mouthpiece of the right instrument, it vibrated the diaphragm and iron armature. This altered the reluctance of the magnetic path, changing the magnetic field in the coil, and inducing an alternating current audio signal in the wire. This passed through the wires linking the instruments and caused a changing magnetic field in the receiver instrument. This vibrated the iron armature and the diaphragm, creating sound waves which reproduced the sound striking the first instrument. Alterations to image: cropped some whitespace out and adjusted brightness and contrast slightly to improve visibility.

Caption: FIRST BELL TELEPHONE RECEIVER AND TRANSMITTER - The receiver is on the left in the picture. A thin membrane of goldbeater's skin tightly stretched and fastened with a cord can be seen over the end of both devices. An electromagnet is also shown over each membrane. This thin membrane, with a piece of soft iron attached, was used in place of the soft-iron disk of the modern receiver