Eddy currents; older speedometers, utility meters, and coin acceptors

From: Nicholas Bodley (nbodley@world.std.com)
Date: Wed Mar 28 2001 - 11:51:08 PST

A person who shuns informality might call them Edward currents. :)

Anyhow, a closely-related principle is the basis for traditional
speedometers; a few decades ago, there were mechanical bicycle
speedometers one might find as discarded/unwanted devices. Likewise
for cars, trucks, and buses.

One design (I don't know the field)(oops, pun...) had a rotating
magnet (all did) shaped like a ring with a gap; it was a variant of a
horseshoe magnet. It was turned by the speedometer cable. Surrounding
it was a shallow aluminum cup with its walls close to the magnet.
(This is called a "drag cup".)

The cup was mounted on low-friction pivots, and had a hairspring to
turn position it to zero. A pointer on one end of the cup's shaft
showed speed. Increasing speed dragged the cup with more torque, and
when the hairspring's torque matched that of the drag, the pointer
came to rest.

I'm sure that tachometers (rotary speed indicators) used this
principle, and there are probably still some being sold of that type

An electric utility meter has a very special electric motor in it to
drive the indicating dials. Its rotor, the aluminum disk, has very
low-friction bearings (one type has a magnetic-attraction suspension
at the top). The field coils (one for voltage, the other (pair) for
current) drive the rotor with a torque proportional to the power
(volts X amperes). Retarding magnets acting on a different part of
the disk make its speed very accurately proportional to power. Older
meters have a very easily-visible magnet; newer ones have much-
smaller ones.

Finally, coin acceptors (P.C. for slug rejectors :) roll coins down a
ramp between strong magnets. If the coins conduct electricity well
(older copper pennies, older silver-alloy dimes and quarters, and
modern copper-center sandwich coins), they roll slowly, and drop
right off the end of the ramp. Coins with poorer electrical
conductivity (nickels) scoot off the end, and fall into a different
hopper, (or used to!). There's more physics and plain clever
mechanical design, such as washer detectors and sizing cradles
involved, but they are not related to Lenz's Law.

Nicholas Bodley |@| Waltham, Mass.
Please reply to nbodley@alumni.princeton.edu
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