I was watching a show last night, 'The White Rabbit Project', and they had a segment on the first music 'streaming' service which worked over the phone lines in NY, NY in 1909. The guy who invented it had the idea to generate the notes electrically so he made us cogs with different numbers of teen which were are rotated on a shaft past pickups. Each cog produced a frequency, i.e. a different tone. This equipment was humongous and the business flopped in about a year.
The Hammond organ came out in 1935 and used a miniaturized version of this idea with many small rotating 'tone rings' rotating on several shafts. So 'tone rings' were really used to produce tones at first. It makes sense then that a cog for sensing the rotation of something on a car was called a 'tone ring'.
