If the two voltages (at freezing and at melting) were different, your mercury is contaminated. To purify it, submerge the test tube in the chilled alcohol and wait until about half the metal has solidified. Pour the remaining liquid into a separate container for waste mercury. Repeating these steps a second time should yield mercury that is at least 99.99 percent pure. Produce enough of it for your needs and then perform the calibration as described.

Once you have determined the output voltages for the freezing point of mercury and for the freezing and boiling points of water, check these numbers against the table below. This comparison will immediately reveal the corrections you should apply to the tabulated values at ¿34.8, 0 and 100 degrees. You can then interpolate the appropriate changes to all temperatures within this range, which brackets even the coldest winter nights that Rhode Island is ever going to see. Now my home weather station is operational again--just as long as I don't ever decide to move to the South Pole.

The Society for Amateur Scientists will offer a kit for this project until December 2001. The package contains a large, hand-blown test tube with a flat bottom, a rubber stopper and a glass thermometer well, a plastic spill guard, protective gloves and an insulated flask for the dry-ice solution. Mercury and alcohol are not included. The price is $150. To order, call the Society for Amateur Scientists at 877-527-0382. You can write the society at 5600 Post Road, #114-341, East Greenwich, RI 02818. To purchase Scientific American's new CD-ROM containing every article published in this department through the end of 1999 (more than 800 in all), consult www.tinkersguild.com or dial toll-free: 888-875-4255.

Further Information:

The Society for Amateur Scientists will offer a kit for this project until December 2001. The package contains a large, hand-blown test tube with a flat bottom, a rubber stopper and a glass thermometer well, a plastic spill guard, protective gloves and an insulated flask for the dry-ice solution. Mercury and alcohol are not included. The price is $150. To order, call the Society for Amateur Scientists at 877-527-0382. You can write the society at 5600 Post Road, #114-341, East Greenwich, RI 02818. To purchase Scientific American's new CD-ROM containing every article published in this department through the end of 1999 (more than 800 in all), consult www.tinkersguild.com or dial toll-free: 888-875-4255.

This article was originally published with the title Calibrating with Cold.

Comments