Why is my multimeter not measuring current?

Question

I'm doing a little bit of experimentation with a glass of water and a 9 V battery. I added a little bit of salt to the water, and I measure the resistance in the water from probe to probe to be ~60 kΩ. Ohm's law tells me the current (given negligible additional resistance in wires, I'm using short copper wires) should be ~150 µA if I connect the terminals of the battery through electrodes separated in the water. When I drop the wires in there I can visibly see small bubbles forming on the electrodes, which tells me that current is indeed flowing, and chlorine and hydrogen gas are being formed.

However, when I then proceed to detach one wire from the battery and connect it to one of the probes of the multimeter, and then connect the other probe to the battery instead (connecting the multimeter in series with the circuit, as you're supposed to to measure current), then nothing happens. The multimeter is supposed to be able to measure to a resolution of 10 µA, so ~150 µA should definitely be within its detectable range. However, it's not just that there's no measurement, but there are no bubbles forming, and thus presumably no current (or at least not nearly as much). I've connected the probes together measuring resistance to ensure that no fuse has blown, and it's measuring only ~5 Ω, so there's clearly a connection, and this resistance should be negligible compared to the ~60 kΩ in the water.

So, why isn't there any detectable current flowing when I'm doing this?

Here's a schematic, as requested by several commenters:

On top is the connection with just the battery and wires in the water, below is the multimeter connected in series with that circuit.

The multimeter is a Biltema 15-133.

Answer 1

Why is my multimeter not measuring current?

Because the internal 200mA fuse is blown. Check the manual, page 4 (på svenska, eller s.7 på norska), section "Byte av multimeterns batteri och säkrningar", for hints on how to access the fuse.

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According to the manual for Biltema 15-133, your multimeter has a 200mA fuse in the low current measuring circuit.

Whenever you set the range dial to one of the following settings:

• 2000μA,
• 20mA,
• 200mA

and there is a voltage source attached to the input terminals, the fuse will be blown unless the voltage source is very weak.

Since this is a lousy design for a meter, I can almost guarantee you turned the range selector through those ranges while measuring voltage at some point, and the 200mA fuse is blown, and that's why the meter doesn't measure current.

As far as I'm concerned, until you verify that the 200mA fuse is not blown, you're wasting your time.

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Reproduce the result below first, without worrying about water and salt. If you can't measure the current in the setup below, there's something wrong with your meter. Most likely a blown fuse or operator error.

Again, forget about water and salt until you can get current measurement to work at the 0.1mA DC level.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

This was an excellent experiment in many ways. Correct observation of basic phenomena, good suggestion of underlying science, correct set up of measurement, diagrams with all essential information, inference that something unexplained has happened, search for explanation.

The first rule of science is actually: reproducible or nothing. As Popper put it: "non-reproducible single occurrences are of no significance to science”. And you certainly can't reproduce this result with lots of meters.

And as a lab maxim: when you changed the circuit to do the measuring, do you actually have the same circuit?

In many labs I've worked in, we used the word "heisenberger" for effects caused by the act of measuring.

heisenberger, n. any surprise change introduced by the measuring equipment, after Werner Heisenberg's principle about measurement altering the observation. Often drawn as 🍔, hence often spelled -urger.

^{simulate this circuit – Schematic created using CircuitLab}

If at all possible, have two meters, so that when you get a surprising result you can cross-check. Which actually is a general rule, especially in electronics: If at all possible, have two of each thing, so that you can cross-check when something surprising happens. Is my battery faulty? Does this wire have a break? Have I blown up this transistor?