full thottle switch

[txrocco at sbcglobal.net (ATS - Patrick Bureau)]

okay could we get over the "this is what I mean" for a moment

I have an miliam meter, 2 leads, the test plugs has 2 prongs....

question: why would it blow the fuse?


ATS - Patrick Bureau - txrocco@sbcglobal.net
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=>-----Original Message-----
=>From: scirocco-l-bounces@scirocco.org
=>[mailto:scirocco-l-bounces@scirocco.org]On Behalf Of T. Reed
=>Sent: Sunday, September 21, 2003 1:44 AM
=>To: L F
=>Cc: scirocco-l@scirocco.org
=>Subject: Re: full thottle switch
=>
=>
=>> 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.
=>
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