:That you fail to find any reference proves nothing.  That you are unaware of any existence is enough evidence for you to decide that it connot possibly exist.  As DieSwartzPunkt says, it is a perfectly logical development prior to Hall effect sensors coming into being.  As he also points out, two people seem to think they did exist, so you are in the minority anyway (but as usual, you would never let that stand in the way of a good deletion).