As a curiosity I measured the voltage on the leads of a tombstone
welder, it's an old Craftsman. I measured about 0.24v to 0.35v depending
on the current setting. This was with no current flow.
I thought it would be higher! Am I missing something? Does the
voltage increase with current flow? (I don't see how.)
It should read what the OCV (Open Circuit Voltage) is which will depend
on your welder specification. Mine is 50V or 80V depending on whether
low or high voltage range setting is selected, it's not a tombstone but
an old UK made Pickhill oil cooled welder.