The Clinical Thermometer

New Zealand Tablet, 27 March 1913, Page 49

The Clinical Thermometer

Little does one think when he is in bed with fever, with a piece of glass sticking out of his mouth. and the doctor waiting impatiently for it, to what great trouble the makers oi the thermometer have gone to make the instrument accurate (says the Madras W atchman). The things necessary in a good clinical thermometer are, first and above all, one that reads accurately; secondly, one that will show the temperature change in the shortest possible time; thirdly, one that will not carry germs; and fourthly, it must be self-registering. ; There are more things in the way of accurate reading than one would think. The glass has to be melted and blown, and some glass takes several months to settle back to its normal size; so, if the thermometer has - been filled and graduated in the meantime inaccurate readings are the natural result. If the bore is large, and therefore holds a comparatively large quantity of mercury, the mercury will be cooled by outside air before it rises to the point it would [otherwise when heated by the mouth. To get a quick reading, the heat has to get to the mercury quickly through the.glass, and as glass is a poor conductor of heat it has to be made very thin indeed for quick results. In order that no germs may be carried, the thermometer is made as nearly smooth as possible and the marks are put on the inside of the glass. - . -. The self-registering device is ingenious. It'shows the highest temperature to which the thermometer has been subjected and does not 'come back' till put back by the physician. Just above the mercury bulb there is a smaller bulb, nothing more than a widening of the tube; above this is a contraction in the glass, making the tube very small, indeed. When the mercury expands, it is forced up through this contraction by the enormous pressure of heat expansion; but when it cools off and starts to come back nothing pulling it but its weight, it cannot come. The physician, after looking at it, generally gets it back by holding it in his hand, bulb outward, and describing a semicircle very quickly with his arm. The centrifugal force here developed is greater than the weight of the mercury, and so brings it back. Perhaps it is not always wise to put too much trust in certain thermometers one comes across in this country of fevers. We have known cases of people giving themselves fever in their anxiety to test their temperature, as we have known other cases of imaginary sickness after assiduous reading of ' family medicine books.' The care exercised by the United States Government, for instance, in the matter of clinical thermometers should arouse great distrust of that much employed register of temperature. The American Government, through its Bureau of Standards, took an interest in the accuracy of clinical thermometers on the market and requested several firms to submit , samples for inspection. All did so, and awaited with anxiety the result of the Bureau's tests. It was found that a large number of the ones submitted did not agree with recognised standards and that their accuracy was therefore none too great. So the Bureau undertook to examine and fully test all thermometers got out, at small cost, and to put on them the mark of the Bureau, which guarantees their accuracy at the time of testing and their continued accuracy within small limits for the rest of the time. Their inability to guarantee accuracy after testing is due to the unknown factor of glass contraction, due to cooling from excessive heat in their manufacture. On the end of every thermometer tested, therefore, are to be found etched in the glass the letters 8.5., and following them the serial number of that particular thermometer; so that whenever it is desirable or necessary one can always see whether or not he is using a Government tested instrument.