Bell's 'Harmonic' telegraph transmitter and receiver, 1873-1874.

Bell's liquid transmitter, March 1876.

Early telephone by Alexander Graham Bell, c 1870s.

Early telephone by Alexander Graham Bell, 1877.

Early telephone by Alexander Graham Bell, c 1870s.

Early telephone by Alexander Graham Bell, c 1870s.

Bell's liquid transmitter, 1876.

Bell's centennial telephone transmitter, 1876.

Early telephone receivers, late 19th-early 20th century.

Jansen's microscope, c 1876.

Wall telephone with Edison chalk receiver, early 20th century.

Advertisement for the telephone, 1877.