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


P J King wrote:
> Hello:
>
> I couldn't seem to find a valid FAQ to this newsgruop (unless the
> Electronics Search FAQ was it, but it doesn't appear to say much), so
> I apologize if this topic has been "beaten to death" in previous
> posts.
>
> I have about 15 multimeters at the school I teach at. There are 3
> different brands (the names escape me at the moment), all of them are
> digital. I set up two small lamps in series, and test the resistance
> of the lamps. Lamp 1 is 260 Ohms; lamp 2 is 340 Ohms. The total ought
> to be 600 ohms.
>
> But when I test the connection across the two lamps, I get 560 ohms.
I
> thought this was a fault of the multimeter, but since I have 15 of
> them, and 3 different brands, each one I used gave nearly the same
> reading. BTW, I made sure that each time, the individual loads were
> measured with the same multimeter as used to measure the total
> resistance.
>
> How is it that two loads hooked up in series give a reading that is
> *less* than the sum of the individual loads? This makes absolutely no
> sense. Calibration can't be the problem, since the same multimeter is
> used for all 3 measurements. The error seems to be independent of the
> brand of multimeter.
>
> 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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