Searching with interest at espacenet advanced search for Norman C. Pickering patents

For example, electromagnetic sound reproducer (moving magnet cartridge?)
https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2538164A&KC=A&FT=D&ND=3&date=19510116&DB=EPODOC&locale=en_EP


It begins by explaining how is stylus tracking force needed