[Techtalk] Soldering irons (OT?)

[Rudy L. Zijlstra]

Eeva Järvinen wrote:

>Well, the expensive stuff is very nice to look at (and to use), but my
>father gave me this lecture about a voltmeter's internal resistance
>(and why it's good to be a large number), and in a similar vein, why
>an ampmeter should have a very low internal resistance.  I remember
>those two numbers are important.
>
>  
>
Well, they are important because you do not want the measuring equipment 
to influence the thing you are (trying to) mesaure.
In case of a VoltMeter you want the internal resistance as high as 
possible because
1) if current starts flowing, the voltage goes lower
2) While measuring you basically causing a short. Graphic example: while 
measuring the voltage on your houses electricity system (about 240V here 
in Europe), you do *NOT* want a sizable current to flow (unless of 
course you are willing to change fuses each time?)

In case of an Amp meter, its the reverse, but the reasoning is simmilar. 
An A meter is put into the circuit you want to measure. In order not to 
influence the current, the internal resistance should be as little as 
possible. This can be risky though to the Amp meter itself, which is why 
many have 2 Amp measuring points: 1 with a fuse, 1 without. The latter 
has a higher accuracy. If you don't have an idea of the current, first 
measure with the protected point. If the fuse blows, its only a fuse.

Regards,

Rudy

>HTH,
>Eeva
>
>  
>