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There are several factors affecting the measurement; the meter most certainly does not have an internal impedance anything close to 10 ohms. Most likely is that you're far overloading the battery - 300 mA is a lot for a button cell or even a small alkaline battery. They're designed for a discharge rate much lower, and their internal resistance increases when you try to pull more current than they can deliver. Even a cheap multimeter is expected to measure 10A without disturbing the circuit - perhaps a 100 mV drop = .01 ohm. Try again using a larger resistor.
Will Rogers never met me.
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That's a great point and helps me to learn. However I am using 2 AA in series to obtain 3v.
I still thought 300mA might be a bit too much to pull but in the spirit of experimentation I rolled on. 
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Carry on! 
Electronics remains my first love, despite nearly 40 years since school! It's fascinating, always...
Will Rogers never met me.
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Quote: So it looks like my meter has around 9-10 Ohms
That's difficult to believe.
A quick and dirty test with my cheap multimeter gave a internal resistance of about 0.2 Ohms while measuring about 0.1 A (2.2 V lab power supply on 22 Ohm resistor).
By the way, is the resistor properly sized (common 1/4W ones cannot stand the 0.9 W of your test)?
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By the way, did you check the battery voltage when it was actually connected to the resistor?
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I measure approx 2.2 volts across the 10 Ohm resistor in my circuit.
I'm assuming that would mean something like:
2.2 / 10 Ohm = 220mA
However, I know when I measure that I only see 157mA of current.
So...
220mA - 157mA = 63mA (which are missing somewhere).
I plug in the missing 63 like so:
.8V (voltage drop) / .063 = 12.69 Ohms
So I'm still seeing a extra ~12 Ohms that I can't account for. That is obviously higher than my 9-10 Ohm extra I was calculating but could be due to rounding.
Does that seem at least somewhat correct?
I'm just noodling my way through this to learn more. Thanks,
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Combining a cheap multimeter and an overloaded battery may lead to all kinds of silly conclusions. If you want to comment on a multimeter, make sure you use a lab-grade power supply and resistor both capable to act nominally in your circuit; and likewise when checking a battery, make sure to use a lab-grade multimeter and resistor. And make sure none of the components involved heats up (which indicates operation outside the nominal operating range), as currently your resistor probably and your battery most likely does.
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That's strange. I would espect about 3.0V in a circuit with only the batteries and the resistor (the multimeter providing negligible effect with its high parallel resistance).
[update]
According to the answer to your post on electronics stack exchange it looks you are asking too much to your batteries (I'm not an expert). So I would suggest you to make a test within limits of both resistor and the batteries (e.g. use a 100 Ohms resistor).
[/update]
What's the power rating of your resistor?
With 2.2V you are still well out-of-range on a 1/4W resistor.
modified 5-Mar-18 2:40am.
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CPallini wrote: With 2.2V you are still well out-of-range on a 1/4W resistor.
All good points. Thanks for input.and you're especially right about power rating of resistor.
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In addition to the other sources of error (I'd wager battery internal resistance is the biggest), you've run into ammeter burden[^]. In most modern digital multimeters, the maximum burden is approximately the full scale on your lowest voltage range, probably something like 200mV. (It measures current by the voltage drop across a shunt.)
Welcome to real-world electronic engineering!
Cheers,
Peter
Software rusts. Simon Stephenson, ca 1994. So does this signature. me, 2012
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Peter_in_2780 wrote: Welcome to real-world electronic engineering!
Thanks
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Years ago, I had a weird problem with a prototype board and slapped a Tektronix scope on to start looking for spurious signals and the like.
And the problem went away.
Finally worked out that the scope probe was smoothing out a high frequency nasty so I contacted Tektronix to find out the probe capacitance - and their suggestion was "ship one of our scopes with each board" ... funny guys, very funny. (They did tell us, and a capacitor added to the PCB cured the problem).
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
AntiTwitter: @DalekDave is now a follower!
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Interesting story.
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As an electronic engineer I welcome you to this world! There is a proverb in German about measurements "Wer misst misst Mist". My freehand translation: If you measure you measure bullshit. There are so many factors affecting your result and you already realized: A breadboard circuit doesn't represent the ideal world of Ohms Law! Resistors have tolerances (do you know the exact value?), your meter has tolerances, your probe has resistance, your wires etc. And you are only in the low DC voltage area! The real fun starts with high frequency, where every cable, every socket etc is a combination of capacitor, induction and resistor.
I suggest you measure the voltage drop over the resistor and then calculate your current. Also try to use a regulated power supply or similar that gives you a constant current. With a 3V battery, your won't have a lot of fun for long and your measurements will be different every time. There are also online circuit simulators (https://www.circuitlab.com/) and SPICE (Online Circuit Simulator with SPICE). SPICE is around for a long time and open source.
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Thanks very much for your feedback. I really appreciate it and agree with all of it.
Especially...
hug.login wrote: If you measure you measure bullshit.
I kind of figured that was what I was doing.
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Hi, The picture on your electronics stack exchange post shows that you are using a solderless breadboard. This is very likely contributing significantly to your "phantom" resistance. I count at least 6 breadboard connections in the picture any one of which could contribute several ohms if you are unlucky!
The contacts in these things are only spring loaded so any oxidisation on the contacts or the wires will contribute additional series resistance.
Make sure that all the wire ends, including the resistor are freshly cleaned and I would also try to remove some of the oxidation from the breadboard contacts by inserting and removing each wire several times.
Better still, take the breadboard out completely and solder the battery and one end of the resistor directly together. Even then you will still have spring loaded connections from the crocodile clips on the ends of the meter leads.
I understand that you are trying to produce something that is reproducible by people without an electronics workshop, but for you tests I would also try to eliminate the battery by using a bench power supply so you can eliminate voltage droop.
Graham
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Excellent advice!
I hate breadboard - but nowhere near as much as I hate wirewrap ... 
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
AntiTwitter: @DalekDave is now a follower!
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OriginalGriff wrote: but nowhere near as much as I hate wirewrap
Just when I hoped you would wrap up my new boards for me...
I have lived with several Zen masters - all of them were cats.
His last invention was an evil Lasagna. It didn't kill anyone, and it actually tasted pretty good.
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Oh I do it from time to time - but I don't like it. The amount of time you spend chasing bad contacts because you wiggled it slightly... 
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
AntiTwitter: @DalekDave is now a follower!
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Great info and I really appreciate it. Thanks very much.
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You can't monitor a system without affecting its behavior.
I'm glad I don't do electronics (my father did).
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PIEBALDconsult wrote: You can't monitor a system without affecting its behavior.
So true.
Luke (PiebadConsult), come to the dark side.
Electronics and open h/w is so amazing now. So much fun. You must get started.
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One other suggestion: I'd be careful putting any multimeter in series to measure current. It's very easy to blow your meter. Yes, I know they have fuses, never the less, caution. Years ago I worked with a seriously overpowered EE. This guy was chasing a "spurious" signal in his control circuit. In attempts to isolate things, he had done all sorts of things to the scope probes, no ground wires, etc.
When he noticed the insulation dripping off the probe wires, it was too late.
At this point, I think he said something like "elephant it" and added a resistor.
Charlie Gilley
<italic>Stuck in a dysfunctional matrix from which I must escape...
"Where liberty dwells, there is my country." B. Franklin, 1783
“They who can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.” BF, 1759
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