In a message dated 98-01-16 23:27:05 EST, you write:

<<
 Dave,

 There is a common type of Hall sensor that *will* detect gaps in an iron or
 steel rotor, e.g. a gear. The sensor assembly includes a permanent magnet to
 bias the Hall device.

 Mike Hardwick, for Decade Engineering
 In Oregon, Land of Magnificent Stumps
 decade@worldnet.att.net ~ www.decadenet.com
  >>
I know.  I have used these before.  They are quite expensive.
The original post just stated a Hall effect sensor.

Dave