Wayne Kerr S121 AF signal generator.

Wayne Kerr Research Laboratories (http://www.waynekerrtest.com/history.php) was founded in London, in 1946, by two ex-BBC engineers, initially producing admittance bridges to test broadcast equipment (see 1984-1663 for an admittance bridge in SCM collections). They moved into other areas of electrical testing and measurements in the 1950s and quickly became a well-respected name in testing equipment. The S121 signal generator was not only used for audio and electrical applications, but commonly used in laboratories in contexts as diverse as physiology and chemical engineering (see, for example, D.A. Haydon and B.W. Urban, ‘The Admittance of the Squid Giant Axon at Radio Frequencies and its Relation to Membrane Structure’. J. Physiol. 1985, 360, 275–291 or John R. Langan and G. Arthur Salmon. ‘Physical properties of N-methylpyrrolidinone as functions of temperature’. J. Chem. Eng. Data 1987, 32(4), 420–422). The S121 was manufactured from the 1950s to the 1970s (TBC by further research) and many are still in use (including the one offered until last year). It is a typical example of the essential but often overlooked testing equipment at the heart of development, maintenance and repair of electronic media technologies in the 20th century and beyond.