Amplifiers before electronics — the Magnifiers

Amazing ingenuity was shown by ‘Electricians’ prior to the development of the thermionic valve (tube) into a reliable means of amplification of extremely low current signals in the earliest telecommunication systems. This paper classifies the devices used and then describes the principles involved in the group known as Magnifiers. Super-sensitive galvanometers preceded signal amplification; the mirror galvanometer and the siphon recorder are therefore also described.