Begin by filling a wide-mouth stainless-steel thermos (the kind used to keep soup warm) about halfway with a solution of 91 percent isopropyl alcohol. (This is the least dilute variety George can buy at his drugstore.) Next obtain a small block of frozen carbon dioxide from a nearby liquor store. (Check the Yellow Pages under "dry ice" to find a local supplier.) Wrap the frosty mass inside a towel and hammer it into small fragments. Transfer some of the chips to the thermos using tongs and stir the concoction with a wooden spoon until it stops bubbling.
Image Credit--DANIELS & DANIELS
Before you go to the trouble of making up the cold liquid, though, you'll need to fashion a "thermometer well"--something to protect your thermocouple when it is immersed in the mercury. You can buy just the right piece of glassware as part of a kit from the Society for Amateur Scientists, or you can make your own from a slender Pyrex tube by following the procedure for fabricating test tubes that was given in the May 1964 installment of this department.
With the appropriate thermometer well and cold bath, a temperature calibration at the freezing point of mercury is easy to perform. Secure a large test tube and a matching rubber stopper from a purveyor of scientific supplies (one is Fisher Scientific; www.fisher scientific.com or 800-766-7000). Make sure the stopper comes with a hole that is the right size to accept the thermometer well, stuck just far enough in that it hangs about two centimeters from the bottom of the test tube. Fill the test tube two thirds full with mercury. Pour some alcohol in the well and insert your thermocouple so that it is about one centimeter from the bottom and, if possible, not touching the glass. Then immerse the well in the mercury and push the stopper snug. Finally, place the entire assembly gently into the thermos containing the chilled alcohol solution.
Be careful! The vapor pressure of mercury at room temperature is sufficiently high that prolonged exposure can cause brain damage. Don't allow pregnant women or children anywhere near, work only in a well ventilated area, and wear protective clothing and safety glasses. Also, keep your containers of mercury tightly sealed. And because spills are notoriously difficult to clean up, think through all the ways that an accident might happen before you begin. Make sure, for example, that you can fully contain any spills if one does take place by keeping the mercury-filled test tube low to the table and over a large food-storage container. Once contaminated, plastic can never be completely cleaned, so clearly and permanently label it "DANGER! MERCURY CONTAMINATED--DO NOT USE FOR FOOD."
I should emphasize that mercury is rightly classified as hazardous waste, and by law it must be disposed of safely. Because regulations vary regionally, you'll have to contact the hazardous materials office of your local fire department for guidelines.
To carry out the calibration, carefully monitor the output of your thermometer as the temperature plummets. When the mercury begins to freeze, the voltage will remain nearly constant and won't drop again until all the mercury has solidified. After you see the voltage fall for a second time, remove the test tube and place it in a sturdy stand. Then record the output of the circuit as the mercury melts. Again, it should linger at one voltage for a while. If the voltage stabilized at the same value as before, you can be confident that the temperature plateau occurred at exactly ¿34.8 degrees C.