Clinical Thermometer

Why doesn’t a clinical thermometer register room temperature when you take it out of your medicine cabinet?

We trust thermometers.

If our temperature is 99.8 degrees, we say we have a fever.

But, when we take out the thermometer, the temperature reading seems to have no correlation to reality. Why isn’t the thermometer sensitive enough to know that the room temperature is really much lower than even 96 degrees, of whatever the lowest number on the thermometer scale is?

In order to understand this amazing phenomenon, we will need a crash course in thermometer anatomy. The metal part of the thermometer that we stick in our mouth is the called the “bulb.” The remainder of the thermometer is known as the “stem.” The mercury flows within a capillary in the glass stem, called the mercury column. This column is very narrow, about the width of a human hair. At the base of the stem, near the bulb, you’ll see a bump, which is called the constriction. (which it really is)

The constriction is the key to how a clinical thermometer really works.

To create the constriction, one spot of glass is heated as create a bump----controlled warping. The constriction works as a physical impediment to keep the mercury from going down towards the bulb until you shake it down. If you don’t remember to shake it down, it will only go to a higher temperature, not down. The only reason any temperature in a thermometer rises is because the mercury in the bulb expands from the heat in your mouth. When the mercury cools and wants to retract, the constriction refuses to allow any downward flow.

If you are a curious customer, and you should take your household thermometer out and examine its packaging, you will probably see a note that indicates that the thermometer “conforms to ASTM E667.” This gibberish refers to the fact that all U.S. manufacturers of thermometers have voluntarily agreed to meet the standards of the American Society for Testing and Materials, an organization that sets the standards for a great many products and services. ASTM is a non-profit educational association, founded way back in 1898, that publishes over 7000 separate documents detailing standards in fields ranging from steel and chemicals to robotics, medical devices, and child-resistant packaging. Committees, comprised of all volunteers, contribute their time and effort to set standards, and ASTM bylaws require that a majority of committee members may not be comprised of producers of the item for which the standards are being set.

Source: IMPONDERABLES by: David Feldman
Copyright @ 1986 by David Feldman Pgs. 35-36.

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