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ancc va)ue at any othcr temperaturę.

We can also compute the resistance by using the temperaturę coefTicient and not referring to Fig. 5 at all. In the example just given. the temperaturę inerease (from 25°C to 50°C) is 25°. Multiplying this by the temperaturę coefTicient, 0.7^r/_c (0.007), gives an inerease of 0.18 ohm for every ohm of initial resistance. Since the initia) resistance was 100 ohms. the new resistance will be (100x0.18) or 18 ohms higher than at room temperaturę.

It can be seen from these properties that the “Sensistor" can bo employed as a sensitive thermometer when used with an ohmmeter. A new scalę for the ohmmeter can be calculated without the laborious process of calibrating it. Care must be taken in applications of this kind, however. to make certain that the measuring current flowing through the "Sensistor” is not sufTicient to alter its temperaturo due to this l'R loss.

For probing into very smali spaces where temperaturę indications are de-sired, Texas Instrumentu has developed a glass-encased probo "Sensistor.” Its tcmperature-resistance characteristics are very similar to the straight "Sen-sistor," but its mechanical construction is quite different. It is madę in the form of a cylinder about half an inch long and less than one-tenth of an inch in diameter. See Fig. 2.

Many applications for the "Sensistor’* are apparent, other than its pos-sible use as a thermometer. For ex-ample, it can be used to compensate for temperaturę changes in devices having a negative temperaturo coeffi-cienl. It can also be employed in tele-metering applications where it is desired to transmit information on temperaturo back to a base location. Other applications include amplifiers, power supplies, servos, and computers.

Low-Temperature Thermometer

Along somewhat similar lines. but operating in a completely different temperaturę rangę, is the germanium low-resistance thermometer (Fig. 1) developed at Bell Telephone Laboratories. Although not on the market as yet, this device is undergoing tests and un-doubtedly will be madę by some inter-ested company in the near futurę.

Extensive work is now being carried out in the field of cryogenics. that is, low-temperature research. Tempera-tures involvcd may bo in the neighbor-hood of the boiling point of helium. 4.2° Kelvin (4.2°C above absolute zero) and may rangę downward to within a fraction of a degree of absolute zero or upwards to around 50°K or so. Tem-perature-indicating devices normally employed to cover this rango aro. in somo cases, bulky and in other cases require repeated recalibration. making thom highly inconvenient to use.

The new thermometer, which is formed from a single crystal of n-type germanium, is extrcmely stablo and. onco calibrated. holds its calibrntion very closely ovon though ropoalodly cyclod from room temperaturo down close to absolute zero.

Fig. 3 shows the generał construction of this devicc and the photograph of Fig. 4 indicatcs its size in romparison with a common pin. It is very smali so as to be useful in places where space is at a premium.

The active element is a hridge cut from a slice of germanium doped with arsenie to make it M-type materiał. It also is a resistance thermometer, mean-ing that temperaturę is indicated by measuring the resistance of the germanium slice. This is done by passing a very tiny current through the slice and then measuring the voltage drop across it. The actual resistance can then be computed from Ohm’s Law: R - BIL

These thermometers have e.\treniely high sensitivity, their resistance chang-ing rapidly with variations in temperaturę. For e.\ample, one unit tosted had a resistance of 0.015 ohm at room temperaturo, 14 ohms at 10°K. and 216 ohms at 2°K. Both the temperaturo coefTicient and the actual resistance vary widely with minutę changes in the amount of doping, making it pos-sible to construct thermometers having any of a wide rangę of characteristics.

It appears that the germanium resistance thermometer will have wide application — from precise laboratory measurement of lowMempcratures for cryogenic and calorimetric work to sensing temperatures in outer space.

Strain Gauges

There is another property of semi-conductor materials which is in the carly stages of exploitation by many lal>oratories hut apparently no com-mercial devices are yet availahle. This property is called piezoresistance—e.\-treme sensitivity of the resistance of such materials to tiny stresses and strains. Such a property makes these materials potcntially valuab)e as very sensitive strain gauges.

A strain gauge is normally madę of wire. It is cemented to a beam, shaft. or other dcvice and will indicate very smali bends or twists by a change of resistance. This resistance change, in generał, is quite smali, requiring rather sensitive instruments if extremely smali motions are to be detected with any reliability.

By using a section from a single crystal of semiconductor materiał, sensitiv-ities of 10 or cvcn 100 times as great as with conventional wire gauges ap-pear possible. This opens up a whole new field of application for germanium, Silicon, and other semiconductor materials.

These materials have other advan-tages over conventional gauges—they are stable over a wide rangę of temperatures and respond accurately both to static strains and to varying strains at frequencies up to the resonant fre-quency of the materiał employed.

Fiq. 8. Hall Toltaqes for typical Hall qenerator for rarious values of conlrol current and for yarious maqnetic field*.

October. 1959

Fiq. 9. Basic diaqram of (A) Hall Ef-fect gyrator and (B) Hall EHect circulaior.

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