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Re: Wollaston wire (was light bulbs)



At 1:40 PM -0800 3/16/98, Stanley A Dodds wrote:

Wollaston wire is made of platinum, as small as 0.5 micron. According
to "Procedures in Experimental Physics" by John Strong (Prentice-Hall,
1944), it was used for string electrometers and sometimes for delicate
fuses.

The manufacturing procedure is clever. A much larger platinum wire is
electroplated with silver. The combination is then drawn down until
the platinum core is the desired diameter. After thorough cleaning the
silver is etched off with chemically pure nitric acid at a specific
concentration to minimize bubble formation and insure uniform etching.
The wire is then ready for annealing and mounting - carefully!

A most venerable reference! I should have known the information
would be in there, and I never looked it up! My information was
evidently quite wrong; this makes sense.

I haven't access to a copy of Strong. does it say that the wire
is replated with silver several times between multiple drawings?
otherwise the plating would have to be much thicker than the
platinum wire itself. The wire I used was much thicker than
..0001" = 2.5 microns. That's also a venerable unit; I'm glad to
see someone use it without apology. If we don't expose students
to microns (and Angstroms, etc.) then much worthwhile literature
will be made unnecessarily difficult for them to read.

Leigh