Most mechanical automotive speedometers use the same copper cup and magnets arrangement! Check out how they used to meter frequency: https://en.wikipedia.org/

Most mechanical automotive speedometers use the same copper cup and magnets arrangement! Check out how they used to meter frequency: https://en.wikipedia.org/wiki/Frequency_meter#Vibrating_reed_meters https://en.wikipedia.org/wiki/Frequency_meter#Vibrating_reed...