I have been using some old multimeters in a high school science lab. We did a current an voltage reading from a AA supply (two batteries) with a variety of resistors. In some cases a 1M resistor.
I noted that the only way to get current readings was to set it on the 10A circuit. OK, I thought, maybe the batteries put out that much current in toto.
But I saw this happen even when the 1M resistors were hooked up -- we would get readings of 2.7 V or so which makes sense (old batteries and all) and then when we measured current we would have to put it on the 10A and get readings like 2.0 A or 1.8. Ultimately, the digits made some sense in terms of Ohm's law, it was just the powers of 10 seemed off. So I wasn't sure if I was just reading the thing incorrectly. (Like, was it actually showing milliamps?)
To give one example: I put a resistor in line with a 3V supply. It's 46.4 Ohms. (The multimeter is an Extech instruments, found in a drawer). I touch the probes to the resistor and get 2.60 V, which seems reasonable enough. I connect the red probe to the 10A input and check current and I get ~1.9 A. But V = IR says that it should be
(2.6V) = I (46.4) --> I = 2.6 / 46.4 ~ 0.05A
We have gotten similar results with other meters, though my "good" one (a new Mestek DM91A) reads 0.04A when it's hooked up to the mA side (if I try to set it to mA range the alarm goes off, with the message "FUSE") and about 1.8 A on the 10A side.
In at least one multimeter I replaced the battery. I tried similar readings with a 965,000 Ohm resistor. I got 0.03-0.04 A with the newer Mestek, and 1.8 A with the Extech, which wouldn't read it when i used the milliamp connection.
I suspect there is some stupid mistake I am making in reading the things, and yes I checked that the positive was to positive and negative to negative. (I grew up with analog meters, and you could just read the scale :-) )
Anyhow I hope people about can help me figure this out.
Thanks in advance.