Regarding John Tuttle's question about a mercury hazard at the Welte
studio: "If the carbon rod was suspended over the mercury until such
time as a note was struck, wouldn't there be a spark when the two
elements came in contact with each other?  And, if that was the case,
wouldn't that create mercury vapor"

It will definitely create a spark some of the time.  The mercury will
combine with oxygen to produce (black) mercuric oxide which is not that
poisonous.

The sub oxide of mercury, (red) mercurous oxide, is quite poisonous
because it is deliquescent and loves to combine with water.  Your body
will absorb mercurous oxide through the skin, mouth, nose and lungs.
Bad stuff, but fortunately it's not formed by the processes associated
with a spark discharge.

During the contact (spark) process, the (black) carbon rods get coated
with a light film of mercurous oxide, which is a crude semiconductor.
Thus, the rods will continue to conduct.

I doubt that this process presented much of a hazard to people
associated with this recording process.

Mercury poisoning is generally associated with mercury compounds having
a reasonable vapor pressure, and also with water soluble compounds that
are ingested or breathed into the lungs.  Deliquescent compounds are
also absorbed by the skin.  Compounds in this category are mostly
organic or quasi-organic.  These kinds of compounds are not formed
during electric discharge in air.

Of course, no designer in his right mind would think of using such a
contact system in today's litigious environment.

Bill Finch