Transformer Tap Query

[Sam__]

This feels like a very basic question that I should know the answer to but I just can't figure it out.

I've pulled a transformer from an old scope. It has a bunch of tap offs, as expected, but one of them is a thick black wire which seems to be connected to no other taps. I've checked multiple times with multiple multimeters and they always read open circuit. I was wondering what it's purpose is?

Thanks in advance.

[JackOfVA]

Some high quality transformers have an electrostatic shield between the primary and secondary windings and the shield is brought out to a wire or terminal so that it may be connected to the chassis ground.

[Sam__]

Thanks for the reply.

I just connected it to earth through a resistor and no current flowed, so it looks like it is a shield.