Multimeter voltage reading?

[Eeyore]

Can someone please explain why I get a voltage reading from multimeter which is not connected to a known earth point?

Across battery terminals - 13.7volts. If I hold the meter earth lead I get 7volts, SWMBO even higher. Touching another part of boat (wooden deck) I get various read-outs. No different with shore power disconnected but a nil reading if I stand on cushion. Same readings with two expensive multimeters on several boats in the marina.

Puzzled!

 

[William_H]

We have touched on this subject many times. A digital multimeter has what is called a high input resistance. That means that in measuring a voltage it takes very little current. Generally about 1/100 of one milliamp. Compared to a moving coil analogue meter that might draw 1 milliamp at full scale deflection. Or a small lamp which might draw 100 milliamps.
Now that means that the input resistance might be around 100 meg ohms for the digital multimeter. That means very little current is drawn when you connect it.
Your body might have a resistance to DC of 100 to 200 kilo ohms meaning that it sees your body as very little resistance to current flow in relation to it's own resistance. So one prod on the positive terminal one in your hand touching the negative battery terminal and you can pretty much measure battery voltage via your body. (no you won't feel anything current being so small). It means the meter can sniff a voltage that is barely there.
Now if your battery negative is connected to electrics and eventually the sea then it is a fair bet your body will be connected to the sea via damp floor or even damp air. Hence if you stand on a cushion you are better insulated so less or no reading.
This high input resistance for the meter is excellent in electronics where some circuits use very little current and high resistance such that touching a low resistance meter to the circuit changes the conditions so gives a misleading reading.
However on a boat it can cause real problems. Your light does not work. You remove the bulb and test for voltage and get 12v???? What can happen is that you have a broken but corroded wire or switch such that the several amps needed for a bulb can not get through. But the tiny current for the meter finds hardly any resistance at all.
The answer for most boat work is to fit a resistor or even a small light bulb across the meter terminals. This makes the meter draw current so gives a more realistic readout.
In a similar way you measure the voltage of a 1.5v AA battery and find it around 1.3 volts. Seems fine but the meter draws virtually no current but as soon as you try to draw current from the battery you find it's voltage very low and it won't light a bulb. good luck olewill

 

[Eeyore]

OK William, I think I understood that! Thanks. As we are in the tropics, I did wonder if it were the humidity around these parts which somehow caused an earth connection through bare feet on the saloon deck. Cheers!

 

[reeac]

I recently saw a related effect when I was changing one of the two 6 volt batteries on my old sports car. I was being super cautious as the batteries are located next to the petrol tank and before attaching the earth lead I measured the voltage between the battery earth and the chassis just to make sure that there wouldn't be a big spark. I measured 5 volts which puzzled me for a long time before I decided that it was a spurious reading due to the resistance of the car circuits with everything switched off being about the same as the input resistance of the meter. I finally connected the battery to the chassis with no no ensuing drama.