W. H. Slabaugh
Oregon State University Corvallis
A Sensitive Inexpensive Thermometer
In thermochemical experiments such as those recently summarized by Neidigl and others, the need for a rugged but inexpensive t.hermometer is overcome by employing a thermistor bridge of the type we have used for several years. The circuitry, as shown in Figure 1, is simply a bridge, the resistance of whose components may he varied over a wide range in order to alter its sensitivity. With t,he 1000-ohm thermistor described here, the sensitivity is about lo/ 0.01 ma, and the resistive heating imparted to the calorimeter is about 14 calimin. This heating effect can be practically eliminated by inserting a momentaryon switch in the circuit,. This also avoids excessive l N ~ H.~A,, ~SCHNEIDER, ~ ~ , H., JOURNAL, 42, 26 (1965).
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TEATEB, T. G.,THIS
drain on the battery by extended readings or by accidently leaving the instrument turned on. I n place of the 0.1 ma meter, a suitable scale on a VOM, VTVM, or other "multimeter" serves well, typically providing a range of about 10" full scale. A pair of connertors is mounted on the box in order to accommodate the external met,er (Fig. 2). The actual temperature can be calibrated with an ordinary mercury thermometer, or thermal changes may be ohserved by using the arbitrary scale readings of the meter. The variable resistance, Ra,permits the selection of temperature ranges down to -10" and up to 100°, aud the resistor R5permits an adjustment of the sensitivit,~. In order to protect the thermistor it is advisable to enclose it in an envelope made from glass tubing and sealed with paraffin or other similar wax. Imbedding the thermistor in paraffin within the glass envelope speeds up the response time and electrically insulates the leads. This type of thermometer adequately withstands the wear and tear of freshman laboratory use, though the battery must he replaced each year.
=5 Figure 1. Circuitry for thermistor thermometer. Parts list: RI and Rt, 1000-ohm 0.5-watt; R1, 2000-ohm voriabie rnirtor or potentiometer; 14, 1000-ohm thermi~tor(such ar GB 31P1, Fenwai Electronics, Inc., Fromingham, Moss.); Rs. 2000-ohm variable resirtor loptionol); S, SPST or momentary-on witch.
Figure 2.
The thermistor thermometer (ports not to scale).
Volume 42, Number 9, September 1965
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