Re: Multimeters (My first post to sci.electronics.basics)

On 23 May 2005 07:03:18 -0700, "Chris" <cfoley1064@xxxxxxxxx> wrote:

Thanks for all your responses, they were a great help!

PJK

>
>Welcome to s.e.b., P.J. - the water's fine here among the groundlings
>and basic electrical/electronics questions are (usually) answered
>without snark.
>
>I'm assuming you have two very small incandescent bulbs. Incandescent
>lamps have a non-linear resistance characteristic. That means their
>resistance increases along with their temperature. In other words, the
>measured resistance will decrease as current (power) used to measure
>the resistor decreases.
>
>Resistance is measured by putting current through a resistor, and
>measuring voltage across it. Resistance is inferred through Ohm's Law
>(R = V / I).
>
>The higher resistance of the two bulbs in series may decrease the
>measurement current from the meter. If that's the case, each bulb has
>less power applied across it, and so it's a little cooler. This means
>lower voltage across each bulb, therefore lower total voltage and lower
>total inferred resistance. That may be why they don't "add up".
>
>Don't depend on accurate resistance measurements of bulb filaments
>unless you take accurate filament temperature into account. And you
>should always keep in mind the effect of the instrument on the
>measurement.
>
>A friend of mine used to call our shop claw hammer his "Heisenberg
>Uncertainty Principle Tube Tester" many years ago. Since he hated
>wasting time swapping out marginal or possibly bad tubes with other
>marginal or possibly bad tubes, he always tested them this way last
>(inside a box in the shop garbage can). He said the advantage of this
>test was, once you were done, you always had a definitive and
>satisfying answer to whether the tube was good or not. And the
>instrument definitely affected the measurement. ;-p
>
>Good luck
>Chris

.

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