full thottle switch

[rocco16v at netzero.net (L F)]

Yes, think we are just dealing with terminology here.  
The sun is coming up....I think I'll wash the Scirocco.
:)
Larry
  ----- Original Message ----- 
  From: T. Reed 
  To: L F 
  Cc: scirocco-l@scirocco.org 
  Sent: Saturday, September 20, 2003 11:44 PM
  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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