full thottle switch - how ammeters work

[txrocco at sbcglobal.net (Patrick Bureau)]

alright you guys lost me a while by I hung in there just in case I would get your wisdom and knowledge from reading your messages but alas I am stumped again...
 
in laments terms please
 
I have a 2 plug connector and 2 leads from the multimeter... how is putting those 2 leads into the plug be a problem ?
 
and if it is not the proper way of doing it, what is ?


"T. Reed" <treed2@u.washington.edu> wrote:
> > By the way, how can a meter NOT receive voltage if it's reading
> amps?????????????? i.e. one > MUST have voltage in order to have amperes.
> Right, Toby? Allyn?
>
> > Larry
>
> ok, larry, heres how it works. its not that the meter must _receive_
> voltage, its that its only capable of _reading_ voltage. when reading
> amperes the input is ran through a *very* low internal resistance (a shunt).
> the meter then internally reads the voltage across the shunt, and displays
> it with a current scale instead. the lower the shunt resistance, the more
> accurate the reading (you want the meter to look like the rest of the
> circuit - i.e. straight piece of wire), so the shunt resistance is usually
> very low (0.001 ohms). so, basically, a meter set to read current is similar
> to running around with a set of jumper cables (well, they cant handle that
> much current, but you get the point).
>
> hth
> Al

Yup, Al's right on the money. It's like measuring the flow through a dam
by just measuring a small portion of the flow through a fish ladder. That
tiny resistance diverts a portion of the current through the meter, while
the rest of it goes right on through.

Which leads us to a rule of thumb: If you wouldn't connect a piece of wire
between two things, don't connect an ammeter between them.. because that's
basically what you are doing. A voltmeter is pretty much safe to connect
anywhere (on your car).

To more directly answer your question, just think of the plumbing analogy
again. A water flow meter (ie. something like a water wheel on the back of
an old-fashioned riverboat) measures flow (amps) but does not cause a
decrease in pressure (volts), whereas a water pressure meter measures
pressure (volts) but does not cause a decrease in flow (it works just like
a tire pressure gauge).

This actually isn't entirely true because the world is not perfect.. so:
You can have pressure without flow (just like you can have voltage at a
battery without anything connected) but in order to measure it you have to
have a wee bit of flow. In order to measure flow you have to have at least
some pressure (think of the zero pressure being zero water in the
plumbing).

As far as meters go..

Analog ammeters and analog voltmeters both use a galvanometer, which is
just a needle that moves a certain amount depending on how much
electricity goes through the coil placed next to it. Tachometers and
speedometers use the same thing (as do ANY electric gauges with needles).

It's just the way the meter is designed (circuit-wise) that is different.
Digital meters are much more complicated (and consequently closer to
ideal) but the idea is the same.

-Toby

_______________________________________________
Scirocco-l mailing list
Scirocco-l@scirocco.org
http://neubayern.net/mailman/listinfo/scirocco-l

ATS - Patrick Bureau - txrocco@sbcglobal.net
'85 2.0L Prowler Orange, Kamei X1 Rocco 
'84 1.8L Silver Restoration and '83 Rocco Trailer Project