Jeff Wisnia wrote:

Ken wrote:

Jeff Wisnia wrote:


Arcane question, and sort of long, but here goes....

Last night, young son was in the process of adding foglights to his
car and asked me for some wire to extend the harness. I wasn't sure
what the current draw would be so I grabbed one of the foglight
assemblies and connected it to the Eico 1050 battery charger/DC power
supply which has been part of my garage tool clutter for nearly 50
years:

http://home.comcast.net/~jwisnia18/temp/eico1050.jpg

When I cranked the voltage up to 13, the bulb lit brightly and the
ammeter on the Eico read around 9 amps. That seemed sort of high to
me so I asked the kid, "How big a fuse is in the harness which came
with those lights?" he looked at it and told me it was a 15 amp fuse.
15 amps for TWO 9 amp bulbs? Obviously something was't right, so I
got my Simpson 260 and saw that the current drawn by that bulb was
really a bit less than 5 amps.

Since SWMBO was out getting some "retail therapy" I had some free
time, and taking the Eico into my workshop, I opened it up,
disconnected the leads to the ammeter and fed it with my bench
supply. That verified again that it was reading almost twice the DC
current passing through it.

By a couple of "cut and trys" I found that about 4 inches of 18 gage
solid copper wire shunting the meter made it read correctly enough
for "gummint work", so I soldered that wire in and closed the Eico
back up.

I believe the ammmeter is what I used to know as a "moving iron"
type, and IIRC the restoring force was supplied by some kind of
permanant magnet field, not by a mechanical spring. Am I right about
that?

I doubt that Eico installed a defective meter when they built the
unit around 1965 (The date marked on the meter.) and I'm guessing
that the meter's restoring magnet weakened over 50 years, increasing
its sensitivity to nearly double.

Anyone have any similar experience with those kind of meters, I'd
enjoy learning more, just for the ****s and grins of it.

Thanks guys,

Jeff


Isn't there normally a shunt across the meter movement? If so, is
it accurate? If it had changed in value that could account for the
increased reading.



There was none across the outside, and the meter was crimped shut, so I
didn't bother looking inside.

But IIRC that kind of meter just used a few turns of heavy wire
connected across it's terminals to create a magnetic field which altered
the total dc magnetic field inside and made a piece of iron on the
pointer shaft change its position.

Jeff
Yep, that is the standard construction; i think a spring was used for
setting / resetting zero position.