A Simple Temperature-Activated SwitchA bimetallic couple switches on and switches off,
depending on the temperature the metals in it see.
If metal A at T swells a little more than B,
then metal B will pull while metal A will push.
On the contrary, if metal B shrinks less than metal A,
then metal B will push and metal A will pull.
So, provided that the two of them are intimately joined,
and provided that the two them are slim enough to bend
(a double-decker foil seems to work about the best)
and provided that a contact is placed facing metal B,
when they're hot, the two switch on,
when they're cold, then they switch off.
However if the contact's been placed facing metal A,
when they're hot, the two switch off,
when they're cold, then they switch on.
So you see it all depends on the thermal coefficient
of expansion and, you know, they mustn't be the same,
or A and B would sit there doing nothing all day long,
and they never would switch on,
or they never would switch off.Karen Tellefsen
kat2@mindspring.com