John S wrote:
Elmer wrote:
This lucrative contract was awarded to Honeywell, a company known for
those ubiquitous, round thermostats containing toxic mercury,

Just about every thermostat manufactured by anyone contained mercury. (The
alternative was bimetallic strips which are nowhere near as precise, and
today digital thermostats have become popular.) But how is that relevant
here?


The mercury in thermostats was not used for temperature sensing. The
"classic" Honeywell thermostats used a bimetallic strip curled into a
coil. The two different metals in the strip expand and contract in
response to temperature changes at different rates from each other,
causing a rotation of the coil.

Attached to the coil is a small, sealed glass bulb with two contacts at
one end. There's a small ball of liquid mercury in the glass bulb which
closes the circuit between the contacts when it is tipped towards that
direction. The rotation of the coil happens very slowly as the
temperature changes, but once it reaches a certain point, gravity
causes the mercury to roll to the opposite end of the glass bulb all at
once. This achieves a quick on-off transition that minimizes arcing or
bouncing.

The set point was adjusted by simply rotating the bimetallic coil so
that the transition occured at a different temperature. It was an
elegantly simple, very reliable design which became the industry
standard for residental and commercial use for many years. The
environmental dangers of mercury have become more apparent in recent
times. In service, they pose no hazard, but items like this must be
recycled in a mercury specific program, and not disposed of into
regular trash.

Alternatives to using mercury in thermostats involved some other
mechanical design that would provide a "snap-action" transition from on
to off at the desired temperature. Mercury was only appropriate for low
voltage, low current switching. Any line voltage thermostat would use a
mechanical contact closure. An alternative to using bimetallic strips
is a gas filled bellows which expands and contracts in response to
temperature changes.

Other companies besides Honeywell used mercury in their thermostats in
both bimetallic and gas bellows designs, but the mercury was still only
used for switching, and not for temperature sensing. Today, solid state
temperature sensors have made bimetallic and gas bellows designs
obsolete, and other electronic components perform the switching
function instead of mercury.

My reason for mentioning Honeywell thermostats in the first place was
because they're a very common household product that almost everyone's
familiar with. It was an excellent product for it's day (although that
was fifty years ago).

Elmer