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.”