26 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



for one complete revolution (360°) of the contact-bearing shaft. The 

 resolution of good potentiometer transducers varies from 0.5 per 

 cent to as low as 0.05 per cent, corresponding to an angular reso- 

 lution of about 2 to 0.2°. The resolution of linear motion potentiom- 

 eters is usually expressed in inches or centimeters; practical poten- 

 tiometers can have a resolution as low as 0.002 in. The resolution 

 of the continuous wire potentiometer, Fig. (1-2) 1, is infinitely small. 

 Increase of the travel length of the contact and of the resolution 

 can be obtained by arranging the resistance and the movement of the 



Fig. (1-2)6. Helical potentiometer (internal view of a Fair- 

 child Controls Corporation type 930 ten-turn potentiometer; 

 by permission). 



contact brush in the form of a helix, as shown schematically in Fig. 

 (1-2)6. Helical potentiometers with up to 40 complete revolutions 

 of the shaft are commercially available. 



The last few turns of the resistance wire on either side of the con- 

 tact travel are sometimes not accessible for resistance variation. The 

 range of angular movement through which a variation of the elec- 

 trical output takes place (electrical angle) is sometimes smaller than 

 the mechanical angle through which the potentiometer shaft can 

 move, although potentiometers can be built with the electrical angle 

 equal to the mechanical angle. 



The resistance increases in general proportionally to the displace- 

 ment or to the angle of rotation. Nonlinear functions between poten- 

 tiometer output and angle of rotation can be obtained in several 



