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Ever Wonder About Thermostats?

Over the winter, we weren't getting heat, so a plumber came to check things out. He looked at the thermostat, the gadget that's supposed to keep the temperature constant. Our problem turned out to be some valve, but I was still astounded to see beneath the simple rectangular cover of our thermostat, a glass container with mercury in it, a metallic coil and a little dial. I wondered what was going on with our steampunk model.

On the front of the device, the adjustable dial, or in my case, lever, controls the temperature at which the thermostat responds by rotating the coil and the glass ampoule that contains the mercury. The setting of a thermometer can put a strain on some relationships.

I originally assumed the mercury would have something to do with sensing temperature, but that turns out to be the job of the coil, which is a bimetallic strip. As the name implies, it is made from two types of metal, such as brass and iron. In response to heat, the two metals expand at different rates. This causes the coil to expand or contract. One end of the strip is secured and the other is free to move. The master clock maker, John Harrison, first developed the bimetallic strip to help make his clock more accurate for measuring longitude. 

As the coil responds to heat, it nudges the mercury switch balanced next to the coil, like parts of some miniature Rube Goldberg device. The mercury is contained in a sealed glass tube with two unconnected electrodes. When the temperature is cool, the coil contracts and the mercury will touch the electrodes and complete the circuit to turn on the heat. And then if the coil shrinks, it shuts off. 

The little dial in the middle is a heat anticipator. It has an adjustable arm to control when the thermostat tells the heating system to turn off. The plumber told me that for a radiant heating system it takes longer to heat up. I'm still not sure whether I did any good by adjusting the heat anticipator.

Nowadays, thermostats are usually digital and I suppose I should upgrade. Mercury is hazardous, and these old-fashioned doohickeys are outlawed in some places. Mind you, choosing a digital thermostat can be an ordeal in itself, though they are supposed to make up for their cost through energy savings. With the milder climate of the Lower Mainland the effect is probably not as great, but I suppose every bit counts.

If you have experience with thermostats, share them in the comments.


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