"Jeff Wisnia" wrote in message
...
Peter Dettmann wrote:

[snip]


Also comes to mind that if there is a magnetic pointer restraint this
is unusual, and perhaps the meter would (as usual) have bal;ance
weights to restore to zero, so perhaps it is calibrated for when the
meter face is vertical (panel mount style)? A moving iron movement
responds to RMS so waveform should not be a problem.

Peter Dettmann

FWIW I just went down and tried an orientation test with the meter
reading 5 amps and the pointer stayed in the same position whichever way
I tipped the unit.


When I worked as a meter tech for EIL many years ago we calibrated iron
vane meters of various types. Most of them had a moveable metal piece that
could be adjusted to affect the calibration. Most of these meters were made
to be opened easily for repair and adjustment and scale changing, but some
(GE I think) had an aluminum band that was swaged on with a special tool to
seal the meter. All iron vane meters I worked on were for AC use, but they
might work on DC. I never tried. However, I think they relied on magnetic
induction which is an AC phenomenon, similar to a motor.

The really cheap meters were Shurite, and they used (I think) a permanent
magnet moving vane in a fixed coil. These were often DC meters used in
automotive and battery charger applications. The permanent magnet might
weaken with age, but I think that would make the meter read lower, rather
than high. If there were no magnetism, it would read zero. All meters I
worked on had springs (taut band types use the band as a spring). A weak
spring could cause a high reading.

The only meters I know to be true RMS are dynamometer types, which use a
moving coil as well as a fixed coil. When the coils are wired in series or
parallel, they read voltage or current. When the coils are separate, the
meter can be used as a wattmeter.

Of course, most such measurement is now done digitally, but it is
interesting and educational to understand the older technology. Also, many
of the lab grade meters were beautiful works of art, with varnished oak or
bakelite cases, and carefully drawn mirror scales to reduce parallax, and
to obtain accuracy to 1/4% or better.

Paul