National Geographic : 1943 Jan
Glass "Goes to Town" The familiar ally of mothers and medicine, shorter than a fountain pen and no thicker than two matches, requires 105 or more intri cate operations, all performed by personal trigger-timing and hairbreadth judgment. Try asking the first twelve persons you talk with, "Why doesn't the mercury in a fever thermometer drop back until you shake it down?" I did, on a bet, and lost. Gist of all the replies was, "Why, I never thought of that." At the Taylor Instrument Companies they have letters of complaint, mostly from young mothers, who write that their thermometers are defective because they won't recede even when they put them in the refrigerator! Of course, if the sprightly quicksilver did drop down to room temperature again as swiftly as it does in other thermometers, a nurse could not chart a patient's fever accu rately. It is kept from doing so by a "con traction," which really is a bulge in the bore -a double channel when observed under the microscope. The mercury column remains stationary at the highest temperature to which it has been exposed after the globules squeeze through the "contraction" for reasons which have to do with the relative energy exerted by surface tension and gravity pull. This highly tech nical point only a physicist can explain-or understand. But the veriest layman is thrilled by the acute skill of the workers who blow the con tractions into slender tubes with bores that may be only one-twelfth the diameter of a human hair. A fraction of a second too long over the flame, and the tiny bore closes. An instant too little, and the thermometer would be a "retreater," which means that the mercury would run back into the bulb when exposed to lower temperature. Fusing the bulb on the end of a tube is an other operation as deft as a surgeon's touch. The tube glows yellow in the tiny point of a blue flame; with a quick, sure motion the worker fuses bulb and tube; a momentary flicker straightens the joint. Timing Shifts from Split Seconds to Months After these more than a hundred operations, from laundering the mercury to shakedown tests and certifying baths, the instrument is far from ready for the drugstore shelf. Nature declines to be hurried about some things. New glass is subject to shrinkage. Especially when, as here, it has been heated and cooled down 16 times, from "cane" to "blank." The manufacturers' time charts jump from split seconds to months. "Green glass" blanks, not yet marked, must be stored in vaults for four months or longer. There the bulb shrinks rapidly at first, then more and more slowly. Not until such sea soning can the thermometer be calibrated precisely for the final scale which indicates body temperature. All these steps in making a clinical ther mometer start with the "canes," or tubing, which is manufactured elsewhere. As passengers speed by the Corning Glass Works plant in New York State, they often ask trainmen about a tall structure that looks like a cathedral tower. That is where much thermometer tubing is born (pages 28, 29). Your delicate bedside thermometer starts as a huge embryo gob of glass, gathered by hand and drawn up inside this shaft as high as 160 feet. In the composite gob already are the clear glass, the white background, the red stripe, even the bore through the center. As the tubing is drawn vertically, the bore gets smaller and smaller, till it is scarcely visible to the naked eye. Lengths of such tubing, or canes, are packed like magnified straws in a giant soda-fountain container and shipped to the thermometer maker. Measuring the Make-up of Air When Torricelli concluded that air must have weight and devised his crude barometer, and his immortal master, Galileo, constructed the first thermometer by sticking a tube from which air had been expelled into a vessel of water, they could not have dreamed of the myriad applications of their inventions. The Taylor Instrument Companies alone make some 8,000 articles having to do with measurements of temperature, air pressure, and humidity. Suppose some super-saboteur smashed all thermometers, barometers, and other glass gauges in the United States. Then factory boilers would burst, bakeries and dairies would halt their flow of loaves and milk, industry would shut down, glass itself could not be made. From bathing babies to melting gum, can ning tomatoes, boiling oil, forecasting weather, airconditioning theaters, tempering steel, or making cakes, precise temperature measure ment and control are vital. Thermometers for vats may be 12 feet long; one which hospital patients swallow is one inch long. Recently a meat-packing plant made a count of all the instruments it used; there were more than 5,000 thermometers alone.