Urban legend, I'm afraid. Disclaiming addendum: with most people and most multimeters! The teacher probably just didn't want you spouting blood all over the classroom and equipment.
Let's look at why:
Using Ohm's law and a pessimistic 1000 ohms for the human, your multimeter would need to put 1000 ohms * 0.05A = 50V across its terminals.
Alternatively, for death by 9V, you'd need to have an internal resistance of 9V / 0.05A = 180 ohms
In other words, unless you're a very good conductor, you're safe.
Phssthpok - firstly, it is current, not amperage. Secondly, most multimeters test with DC, so you'd still be OK. Thirdly, the bits of the sinoatrial node that matter are small. Really small. To get 100mV potential difference across them, you'd have to apply 9V across just 90x their width, assuming homogenous resistance.
So even if a multimeter wouldn't kill in the same way as an electric chair, it could still conceivably kill. I haven't heard of this actually happening though.
My grandfather used to have an old megger (megaohmmeter), probably from the mid-1950s. This meter was from a time long before the VTVM and very long before the FET-amplified meter, and used a 500 volt hand-crank dynamo to generate the power needed for a resistance test! I once experimentally touched the terminals on it while cranking, and it was quite uncomfortable.
I have no doubt that the voltage from this meter, applied in the proper fashion, could interrupt the beating of the heart. Of course, I'm fairly sure that it would have no effect on me, though, as I've taken a blast of 480 volts AC with no noticeable ill effects (aside from two small surface burns... entry and exit). That which does not kill me, makes me stronger.
The basic warning: Unless you really want a Darwin award (our school, best and brightest the Navy has to offer, produces an unfair share), should you desire to measure your bodies resistance do so on the OUTSIDE of your skin. With the legend that once upon a time, a prior student killed himself (intentionally?) by stabbing his wrists with the Fluke 77 leads.
So I went ahead and looked up a functional block diagram of the Fluke 77 III (conveniently available on the web site...). The function for measuring resistance uses two paired transistors to regulate input voltage to between 7 and 9 volts. Thus, at e-troon's suggestion of 1Kohm blood resistance, our Fluke would put a current of no more than 9mA through the heart. Could probably cause a flutter, severe discomfort, and mild symptoms of electrical shock. But unless there was some preexisting medical condition it would not cause death.
That being said... I can think of a few ways to kill yourself while using a multimeter. If for instance you try to measure an excessively high voltage with a standard meter... say, 4400 volt 3-phase AC... you will likely die. But then the meter has little to do with that.
Alternately, if you trust a meter with your life and lose, ie, ignore low battery warning, you can kill yourself. My favorite though, give your lab partner the wrong values, get both rolled back in school, and wait for him to kill you the old fashioned way.
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