(Richard Casady) wrote in news:48b2dc91.755265
@news.east.earthlink.net:

I have heard of AC clamp on meters that form a transformer when in
use. This doesn't seem to work with DC.

Casady



These DC ammeters are very simple devices. Inside a non-magnetic case, now
usually plastic, there is a small bar magnet set off 45 degrees from the
plane of the cable clamps you snap over the heavy battery wire. With no
current flowing, two hairsprings hold the meter at center scale, which is 0
amps.

When current starts flowing in the wire, the resultant steady state
magnetic field around the wire pulls the magnet one way or the other,
depending on how high the current in the line is, which is sort of
calibrated on the scale either side of center zero amps.

The springs return it to zero when the current is gone pulling it off.
Current flowing in pulls it one way, out pulls it the other....

There are no electronics at all in this device. There are no direct
connections, either, so you can measure the current in the cable anywhere
you need to.

http://www.stuttgartperformanceengin...veammater.html

Zero center, old fashioned car ammeters work exactly the same way, but the
current goes through a metal strip isolated from the case through two
terminals on the back of the meter.