A multimeter is a device used to measure resistance and voltage in an object. To do this it emits a small amount of voltage itself.
If you decide to put one cable into your blood stream on one arm, and the other cable into the other arm, it will result in your immediate death. This can be done by merely ramming the probes into veins, or pricking your thumb and tapping the blood on each arm with the probes.
The reason this works is it takes very little electricity to stop your heart, and a multimeter has more than enough. The current travel from one arm to the other via the blood stream and thus, through your heart. Badaboom, you're dead. Dead as a door nail.
How did I come across this knowledge of lethal tools of science? In my technology class at school one day, we were using multimeters to test batteries and everything.
natural.resonance and I decided to see who had the higher resistance by placing the multimeters probes on our temples (no this story does not result in the death of myself nor natural.resonance, if you're wondering)(I had the higher resistance if you're wondering that as well). Anyways, the teacher comes up and informs us not to test our blood.
Naturally, the first thing I thought of was he meant if it was like laying all over the table. I assumed it would like make it change colors or some cool effect, so was about to open a wound when he told us the real reason.
Apparently, some kid in another tech class was playing around with a multimeter before class, and decided he would test his insides. Upon testing this, he...uh...well, anyways...
So that's pretty much my story.

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.

As I understand this, it's not the amperage that kills, but the fluctuations involved. Under normal circumstances, the Sinoatrial node puts out about one pulse every second, causing every muscle cell in the ("lub") atria and then ("dub") ventricles to contract. Sending an electrical pulse of about 100 mV through these cells before they're done recharging from the last heartbeat could cause entire heartbeats to be missed, or cause all four chambers to contract simultaneously so that almost no blood is pumped. Enough of this sort of thing, and death is entirely feasible.

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.

There are two ways a multimeter can kill you, both of which are usually caused by human error:

Failing to protect you from a high voltage it is measuring

When using a multimeter to measure high voltage, it is vital (literally) to ensure that:
  • The multimeter is rated to measure voltages in the range you expect to be measuring (mains voltage, for example, is around either 220v or 110v)
  • The multimeter probes are rated to measure voltages in the range you expect
  • There are no uninsulated parts anywhere between the tip of the probe and the multimeter.
Safety features on modern probes include wide insulated 'collars' on the probes to ensure your fingers don't slip and touch the business end, interlocking insulation on the plugs to ensure that there is no exposed metal where the probe plugs in, and fully moulded fittings to lessen the chance of the wire working loose. If your multimeter doesn't have these features, consider replacing it before working with high voltages.

Failing to warn you of a high voltage it is measuring

When measuring the voltage of a dc source, a digital multimeter will show a number corresponding to a variable reference voltage generated inside the meter. If the source is truely dc, the reference voltage can be accurately compared to the input voltage, and will 'home in' on it, until the number displayed is equal to the input voltage. If the input source is ac, the refererence voltage will keep varying to try to match the input, and the multimeter will show either the average voltage, which is zero, or some random number. Assuming a wire is safe on the basis of this number could be a mistake1. In ac mode, the input voltage is rectified, and the resulting dc voltage is measured. Calibrated properly, this gives a good approximation of the ac voltage.

The other easy mistake to make is to use the wrong voltage range. If the range is set to high (say 5000v), the voltage measured may be too small (compared to the maximum voltage) to register on the display. Likewise, if the range is set too low, any voltage higher than the range will overflow it, and not be displayed. Depending on the multimeter, it may display nothing, 1, or 'high'.

More stupid mistakes include not setting the meter to measure voltage, using a multimeter with a blown fuse, and using the wrong terminal for the live probe (many multimeters use different terminals depending on whether they are measuring current or voltage. It's also common to have different terminals for high and low voltages. Be sure to read, understand, and follow all the instructions that come with your multimeter. Knowing how to use your multimeter safely will greatly reduce the risk of injury.)

Multimeter safety checklist

  • Is your multimeter rated to deal with the voltage you intend to measure?
  • Are your probes insulated all the way from the tip to the multimeter?
  • Is the casing of the multimeter undamaged?
  • Does your multimeter work? (Try measuring a low voltage like a battery first.)
  • Is your multimeter set to measure voltage?
  • Is your multimeter set to measure AC or DC?
  • Is your multimeter set to measure an appropriate voltage range?
  • Is your live probe plugged into the appropriate terminal?
1 - I did this fitting a light socket, and was fortunate enough to fall off the chair I was standing on, breaking the circuit. It was not a pleasant experience.

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.

I was given this warning recently in my Electronics Fundamental classes. Very safety conscious school, and in the US Navy urban legends abound.

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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