full thottle switch

[treed2 at u.washington.edu (T. Reed)]

> Toby, are you SURE it's incorrect?  I thought this is exactly what you
> were saying all along?
> You are saying that an ammeter DOES create a short?     If it does,
> I've been doing it all wrong (with good results) for an awfully long
> time.  (and I have yet to blow a fuse in my multi-meters)
>  Please elaborate...
> Larry

Okay.. I think I see what's going on here (especially since you say you
are getting good results). I think we're hooking these meters up the same
way but talking about that 'way' differently. In my definition of the word
short, you are shorting something if you are connecting a non-resistive
wire across it.

In your definition, you are connecting something to ground. For example,
when you are taking an alternator off and you accidentally bridge the +
terminal on the alternator and part of the engine or chassis with the
wrench and get that nice surprise spark. (never done that before!) That
example is also covered under my definition, though.. since you are
connecting a non-resistive load (the wrench) across the car battery.

I can see how you would apply your definition: you most often connect a
voltmeter  between some positive circuit and ground. Therefore you are
"shorting" that circuit to ground with the meter. This is one way of
thinking about it.. but in my view it is incorrect since you are looking
at the physical connection of the meter, not at what is going on inside.

In a voltmeter, the resistance of the meter is extremely high (~1 megaohm)
so very little current goes through it. Think of it as a test light that
uses almost no power and provides the side benefit of telling you how much
voltage difference there is between two points, not just that there is
enough to light a bulb. Because so little current is going through it,
saying that is causes a short is incorrect. A short circuit implies
infinite (at least in the ideal case) current.

In an ammeter, the resistance of the meter is very low (ideally, it's
zero) so as much current goes through it as you like. If you connect it in
series with a light bulb and a battery, the amount of current going
through the light bulb will also go through the meter (current is uniform
through a closed loop) so the meter will tell you how much current is
going through the light bulb. If you just connect an ammeter directly to a
battery, as much current as the battery can generate (in the ideal case,
infinite current) will pass through the meter. This is a short circuit and
is damaging to (at the very least) the wiring (including the sacrificial
piece of wiring, the fuse) and the battery (it's really not designed to
produce that much current explosion-free).

I think we probably have a better understanding of each other by now.. but
please tell me if I interpreted you wrong.

-Toby

>    His message said something
>   to the effect of "a voltmeter causes a short, not an ammeter", which is
>   incorrect.