Tony Hwang wrote:
Dave M wrote:
It's the weight of the mercury that moves the bimetal coil spring
such that it takes a higher temperature to bend the bimetal coil
spring so that the mercury flows back to the other end of the bulb. When
the mercury moves to one end of the bulb, it makes the spring
reposition itself so that it takes a much higher (or lower,
depending on heat or cool mode) for the spring to return back to its
original position. Here's a link to a web page that describes the
operation very well,
saving me a lot of typing.
http://inspectapedia.com/heat/Thermo...e_Response.php

Cheers,

Dave M

I think you are thinking backward. Spring controls the position of
bulb depending on temperature. Not the other way around.


Kirk Landaur wrote


Yep, I understand that the "Spring controls the position of bulb depending
on temperature". That was, I thought, my intention to describe. When the
spring bends far enough to tip the bulb, the weight of the mercury in the
bulb swings the bulb a bit farther , requiring the temperature to cause a
greater swing in the other direction to make it switch back. That's where
the hysteresis comes from. That was the question from the OP, which is what
I was trying to answer.

cheers,
Dave M