Recently I was toying with a digital multimeter and observed the following. The multimeter was set to measuring AC voltage with the upper limit of 750 volts. It has two probes which I will call "red" and "black" to better illustrate what happened. Both probes were connected to the multimeter and I didn't alter their connection to the multimeter.
I inserted both probes into mains (220 volts in my region) outlet - one probe per outlet hole - and it showed something around 220 volts. Now I unplugged the "red" probe from the outlet while leaving the "black" probe plugged into the outlet - the multimeter displayed zero. I plugged the "black" probe back into the outlet and unplugged the "red" probe from the outlet - the multimeter displayed about 7 volts.
So again, one probe was connected to the multimeter and the outlet terminal and the other was only connected to the multimeter with one end and the other end of the second probe was pointing in the air - not in contact with anything. The multimeter displayed 7 volts.
Why did the multimeter behave that way? Where do those 7 volts come from?