. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. MEASUREMENT AND CONTROL OF TEMPERATURE measurement current through the thermometer wire will heat it unless it is very small, the temperature measured will in fact be that of the heated wire less the cooling effect of its surroundings, and it may well heat its surround- ings appreciably. This current must be kept to an absolute minimum. A typical thermometer has a resistance of the order 100 Q, with an order of 0-4 n/degree C change. Readings to 0-01 Q (0-025°C) should be quite straightfor
. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. MEASUREMENT AND CONTROL OF TEMPERATURE measurement current through the thermometer wire will heat it unless it is very small, the temperature measured will in fact be that of the heated wire less the cooling effect of its surroundings, and it may well heat its surround- ings appreciably. This current must be kept to an absolute minimum. A typical thermometer has a resistance of the order 100 Q, with an order of 0-4 n/degree C change. Readings to 0-01 Q (0-025°C) should be quite straightforward, with the precaution that a voltage of 1 V across the element will cause it to dissipate 10 mW—a fine coil could easily rise TC when passing this current—and the actual temperature in the element will be different according to the thermal capacity and rate of fluid flow past Figure bridge circuit of a resistance thermometer (RT) The resistance in series with the element must be at least of 1 kD, or larger, in order to limit the current in the element to a negligible heat effect ( less than a mW). The support for the very fine wire must also be minimal in size—mica sheet is suggested. Calibration against a mercury thermometer in a gas needs an extremely long time for the mercury to equilibrate; it is possible to calibrate in a bath of pure mineral oil provided: (1) it is ascertained that immersion has no effect on the resistance of the element and (2) that it is cleaned with chloroform and pure alcohol afterwards. Conclusion—The resistance thermometer is an excellent accurate method for measuring gas temperatures and for following quite rapid fluctuations of temperature in them. The application of a seff-balancing bridge would enable it to be used with recording gear, though it is rarely used as a control element. The thermistor—This is a resistance thermometer of a very different kind which merits special treatment. It consists of a minute bea
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