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.

Try the same test with a pair of 220 or 330 ohm resistors. I suspect that the meters pass enough current through the filaments to change their temperature, raising their resistance above the room temperature value. Testing two in series lowers that current so they heat less, and each have lower resistance. Resistors should not have this high temperature coefficient and should not show the effect.

.

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