People sometimes wonder how sinks and tubs become energized when old knob and tube starts to fail.
This is just one of several possible ways:
The deteriorated knob and tube neutral wire, which usually runs under the tub access touches the copper plumbing.
It gets wet over the years and develops electrical potential between the conductor and plumbing.
Because knob and tube connections get loose and fail over time, the plumbing now offers a more direct, low impedance path to ground than the neutral.
The result is people getting a tingling sensation ( or a solid shock) when they take a shower.