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ID number:  BIRRC-P0659
Institution:  Research and Cultural Collections
Named collection:  Historic Physics Collection
Title / Object name:  Coherer
Object type:  Electrical instrument
Date made:  Probably 1890s
Materials:  Glass tube containing iron filings
Measurements:  L 7.5 cm, diameter 0.8 cm

In the early days of wireless a device was required to convert a radio frequency pulse to a d.c. current which could operate a galvanometer or other recorder. This device was called a coherer and one of the earliest forms was, like this example, a glass tube containing iron filings with two leads projecting into them. An r.f. pulse produced a drop in the resistance between the leads. To restore the device for the next pulse it had to be tapped mechanically. A handwritten letter from Sir Arthur Vick dated February 1989 says “This coherer was probably made for Sir Oliver Lodge, though I have no proof of this. It certainly dates from his time.”

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