English: An early telephone transmitter (microphone) invented by US scientist Alexander Graham Bell around 1876. An improvement over Bell's "gallows" type instrument of 1875, it was Bell's first device that could transmit intelligible speech. It was used with his Iron Box receiver in a crude telephone system demonstrated in June 1876 at the Centennial Exposition in Philadelphia which first attracted attention to Bell's work.

It consists of a horn shaped mouthpiece (black) with a parchment diaphragm stretched over the end. In the center was glued a piece of sheet iron. Immediately in back of the iron was an electromagnet (right) whose winding was attached to the binding posts. When wired in a circuit with the receiver and a battery, the current from the battery passed through the electromagnet, creating a magnetic field. When someone spoke into the mouthpiece, the sound waves vibrated the diaphragm, moving the iron armature back and forth. When the iron was nearer the electromagnet core it increased the magnetic field slightly, whereas when it was further away it decreased the field. This changing field induced a tiny alternating electric current in the wire by electromagnetic induction. When the current passed through the receiver in another room, a similar driver mechanism made its diaphragm vibrate, reproducing the original sound waves.

Alterations to image: Increased brightness, and cropped out dark confusing low-contrast background to improve clarity

Caption: Bell's single-pole membrane telephone. Demonstrated before the judges at the Centennial Exposition, 25 June 1876. This instrument was used as the transmitter, the receiver being the Iron Box instrument shown in fig. 8