@antedeluvian - were you thinking of Eddy Currents? Electricty meters have a voltage coil and a current coil acting on the disc, which gets eddy currents induced in it. The interaction between the magnetic fields of the V/I coils and the eddy current in the disc causes it to rotate.
I work for an electricity utility and any disc meters that go wrong are replaced with electronic ones. Divider on the voltage, CT on the current and the rest is done by the processor, displayed on an LCD and downloaded to the meter reader via IR LEDs used as an isolated data link. Usually a backup battery in case of power failure. But they rarely fix them when they go wrong, just chuck 'em.
Used a MR sensor a decade ago in a compass chip , absolutely awful for anything analog, the scale factor, linearity and offset would change all the time, so needed to reset all the time .
One advantage of MR (for on/off applications), is that the magnetic field sensitivity is in the plane of the chip, where as hall effect is perpendicular to chip. More useful being oriented this way with SMD sensors for shaft encoders, or current sensors when placed over a conductor. In both these applications a hall sensor would read nominally zero as its aligned with the null in the field.
Need more bandwidht than a hall sensor or 1-5 Mhz consider magneto resistive. Same techology used in many harddrive read heads. Works like a hall sensor in that they need a bias current to read the restance change typicly aranged in a bridge. Due to the fast response time you can pulse the current to save power. Use two in oposition to cancel stray magnetic fields. Note they can be magntized by large fields most have a reset winding. Made by a number of outfits. Have fun.
My Mom the Radio Star Max MaxfieldPost a comment I've said it before and I'll say it again -- it's a funny old world when you come to think about it. Last Friday lunchtime, for example, I received an email from Tim Levell, the editor for ...
A Book For All Reasons Bernard Cole1 Comment Robert Oshana's recent book "Software Engineering for Embedded Systems (Newnes/Elsevier)," written and edited with Mark Kraeling, is a 'book for all reasons.' At almost 1,200 pages, it ...