Back to Ganoksin | FAQ | Contact

Ring sizing; dissimilar metals


#1

G’day there have been several posts from folk with rings composed or
gold and platinum, both metals making a continuous circle. These
two metals have different coefficients of expansion; that is, they
increase in length (and width) for different amounts for each degree
rise in temperature. Thus one wants to get longer than the other,
but it can’t because it is irremovably joined - the result is that
the combined metals curl as the gold expands more than the platinum.
Thus if the platinum band were to be on the inside of the ring,
heating a cut ring would cause the gap to close.

This property of different expansion of metals is exploited by a
type of mechanical thermostat. Sheets of iron and brass are
scrupulously cleaned and are hot rolled together, then cut into
strips. One end is bent into a U, one arm about an inch, the other
about 5 inches. The short arm is screwed to a piece of rigid
plastic and the other has a a silver rivet as an electrical contact,
which can meet a similar contact fastened to the plastic. This
simple device is sensitive to about 0.5C. and is a very cheap method
of controlling aquaria temperatures, etc.

When re joining a dissimilar metal ring I would suggest the ring is
clamped tightly closed by means of a jubilee hose clip, but as these
clips are generally zinc or cadmium plated, a piece of asbestos paper
should be used to separate the ring from the hose clip. Stainless
hose clips are available these days. Incidentally, I am talking
about the type which has a worm screw drive to open and close them.
Hoping I haven’t added to confusion, – Cheers for now,

John Burgess; @John_Burgess2 of Mapua, Nelson NZ