TRANSDUCERS 



makes the potentiometer extremely accurate at the eight points, and also 

 gives a great improvement in the accuracy elsewhere. 



All types of potentiometer are available with non-linear 'laws' ; thus the 

 resistance may be made to vary as, for example, the square of the shaft 

 rotation. The accuracy with which these functions can be reproduced is 

 usually rather lower than that of the corresponding linear-law potentiometer. 



Potentiometers for linear operating movement can be constructed for 

 special applications using carbon tracks, wound cards or even very fine 

 single resistance wires. Their characteristics are dependent entirely upon 

 the precision of workmanship, though such potentiometers are never likely 

 to be as accurate as the corresponding commercial rotary units. 



The use of potentiometers as transducers is limited to those occasions 

 when considerable mechanical power is available. The potentiometers 

 described may require a driving torque of 10^-10^ dyne-cm (10-100 g wt-cm), 

 although units of special design are available with a driving torque of v/ell 

 under 10^ dyne cm. Furthermore, if complex and inefficient linkages are to 

 be avoided ample movement must be available. Potentiometers require 

 positive drive in both directions, so that in certain cases a return spring 

 may be necessary. 



Liquid potentiometers — The disadvantages of large force and movement 

 inherent in track potentiometers may be avoided by using a liquid column 

 as the resistive element. The moving contact is merely an electrode moving 

 in the liquid, and the forces on it are negligible. Minute electrode movements 

 can be used, as the potentiometer can be made on a very small scale. 



It is impracticable to construct a linear variable resistance (a 2-terminal 

 device — as distinct from a potentiometer, a 3-terminal device) using a liquid 

 element. Unless the cross-section of the electrodes is the same as that of the 

 tube containing the liquid the variation of resistance with electrode distance 

 is very non-linear and almost impossible to calculate. With a pair of small 

 electrodes almost all the voltage drop occurs within a small distance of the 

 electrode surface, and it will be found that the electrode spacing has but 

 little effect on the resistance between them. On the other hand, with the 

 potentiometer configuration, large fixed electrodes fitting the whole cross- 

 section of the container can be used, with a very small moving electrode 

 acting virtually as a potential-exploring probe. If it is not practicable to 

 use fixed electrodes covering the whole area, a linear voltage/distance 

 relationship is still obtained in the centre, provided that the distance between 

 the electrodes is large compared with their dimensions. In this case, of 

 course, only a fraction of the voltage applied across the fixed electrodes is 

 available as output. 



It is desirable in order to avoid polarization of the electrodes to use 

 alternating current with a frequency of at least 250 c/s for energization. 

 Care should also be taken with the nature of the container, electrolyte and 

 electrodes, to avoid chemical action, evaporation or other deterioration. 

 Platinum electrodes are usually safe, while polythene, Perspex and glass are 

 suitably inert materials for the container. Electrolytes which have been used 

 include glycerin, and, for sealed-off transducers, a mixture of hydrochloric 

 acid and alcohol. 



The mechanical input impedance of such a transducer is dependent almost 



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