Should you have an interest in any technical subject ( metal detectors for example )
there is no better source of information than a patent office. All developed countries
hold vast numbers of patents and usually supply copies at modest cost. If you can manage
to visit a patent office ( they are large buildings ) you can look at the patents at you
I have visited the British patent office at 25, Southampton Buildings, Chancery Lane, London, WC2A. And found it to be a most fascinating place.
British patent no 5518 One of the oldest M/D patents.
British patent no 1548239 An excellent patent complete with circuit diagrams and much detail.
U S patent no 4,024,468 Describes an induction balance M/D with inverse discrimination. Includes circuit diagrams and much detail.
U S patent no 4,514,692 Describes motion detectors with much detail but no C/D.
British patent no 1 315 684 describes pulse induction M/D.
U S patent no 4,096,432 Arado patent.
U S patent no 4,128,803 The original motion detector patent with much detail but no C/D.
British patent no 2 004 069 Describes dual frequency M/D.
Other metal detector patents of unknown content.
US 403 0026
US 412 8803
US 424 9128
US 429 3816
US 478 3630
US 486 2316
US 486 8910
UK 154 8239