The
Liquid Telephone (A) that carried the first human voice on 10 March 1876 had a small diaphragm at the bottom, which moved when someone spoke into the funnel. The diaphragm was attached to a wire floating in an acid-filled metal cup. A wire attached to the cup in turn was connected to a distant receiver. As the wire moved up and down it changed the resistance within the liquid, changing the current which was sent to the receiver, causing its membrane to vibrate and, thereby, 
produced sound. 

Bell soon improved upon the first telephone–built largely by Thomas Watson–by using an electromagnetic transmitter, a metal diaphragm, and a permanent magnet. Bell used this set for his first public demonstration in the same year, 1876 at the Centennial Exposition, which made emperor Dom Pedro of Brazil to react, “My God, it talks.” The telephone is now
known as Bell’s Centennial
Telephone (B)

A commercial set was built in 1877. This is popularly known as the
Box Telephone (C). The opening served both as receiver and transmitter. 

On 27 April 1877, Thomas Alva Edison filed a patent for an improved transmitter, which was challenged, but he started producing it.

The 1878
Butterstamp (D) for the first time combined the receiver and the transmitter into one hand-held unit. 

Bell combined carbon transmitter built by Francis Blake with the Bell receiver. The result was the Magneto Wall Set. It was commercially produced in 1882.

Earlier long distance talking required different types of telephones. Like the
one shown (E). It was built in 1886.

With common exchange battery, telephones became much more compact. One of the early
Common Battery phones (F) in 1896.

In 1896, with the invention of automatic exchanges, dialling telephones made their debut. The picture shows a
model (G) introduced much later in 1921. 

In 1964, AT&T introduced the first
Touch Tone telephone (H). 

In 1987, Bell Labs introduced
Cordless Telephone (I) with no difference in sound quality.