I'm restoring an upright Steinway Welte Original with a dandy open
mercury switch.The mercury wasn't there when I got the piano and I
faced the standard dilemma of originality vs. safety.  My compromise
was similar to Thomas Henden's suggestion -- I found an old glass-
enclosed mercury switch, wired it in, and fashioned a little clamp to
hold it safely to the body of the original switch.  It works just fine,
is fully safe, and I think preserves the integrity of the instrument
because not only does the original switch remain, the operating
principle is the same as well.(Plus I get a little ego-trip glow when
I think how smart I was!)

Michael Woolf¶
Wellington¶
New Zealand