Brown repeater, c 1918.
© Science Museum / Science & Society Picture Library
The Brown repeater was devised to strengthen the pattern of telephone signals by copying them onto another circuit. By sending incoming signals through two wire coils, the Brown repeater produced different magnetic fields. These newly created magnetic fields then vibrated a small metal strip or 'reed' attached to a bras cylinder. The cylinder, which contained carbon granules of the type also used in telephones, altered the strength of the current and therefore amplified the call. The Brown repeater was installed at telephone exchanges to boost long distance calls.