Sec. 1-2] MECHANICAL INPUT TRANSDUCERS 29 



potentiometer is limited primarily by the mass of the moving part. 

 In a circular potentiometer the dynamic response is limited by the 

 moment of inertia of the rotating part, which is in general between 

 0.08 to 0.3 oz-in. 2 (15 to 60g-cm 2 ). Special potentiometers are avail- 

 able with a moment of inertia as low as 2 x 10 -4 oz-in. 2 (0.037 g-cm 2 ) . 

 The highest frequency response is in general around 3 cps. 



Noise in potentiometers arises from several causes. 1 When the 

 brush moves over the wire, the electric contact area varies in irregular 

 fashion. This resistance variation gives rise to a noise voltage, not 

 only when a current is drawn from the brush, i in Fig. (1-2)10, but 

 also because a current i b passes from one 

 turn of the resistance wire through the 

 brush to the next turn. Noise can also 

 arise from tribo-electric effect, i.e., a dis- 

 continuous electromotive force arising 

 between two dissimilar metals in a rela- 

 tive motion. This noise voltage can reach 

 values as high as 1 mV, but with the proper Fig. (1-2)10. Current-flow 



„ i •< i i diagram between two wind- 



selection of contact metals, it can be kept ings and a contact bmsh in a 

 below 10 /uY and is usually negligible. The potentiometer, 

 two types of noise described above can 



sometimes be reduced, in experimental work, by the application of a 

 very small amount of petroleum jelly or kerosene over the contact 

 area. Noise, furthermore, stems from the discontinuity occurring 

 when the brush in moving makes contact with a turn of the resistance 

 wire and breaks contact with the preceding turn (resolution noise) . 

 Foreign particles getting between the contacts and oxide films or 

 chemical alteration of the wire surface can cause considerable noise. 

 A very serious source of noise can be contact chatter caused by a too 

 fast movement of the brush over the contact area. 



The temperature coefficient of potentiometers is in the order of 

 0.002 to 0.015 per cent/°C. Because of the use of different metals for 

 the resistance wire and the brush, thermoelectric forces are likely to 

 arise in potentiometers. 2 



The main advantage of potentiometer transducers is their accuracy 

 and their simplicity of operation. A voltage source and a meter are in 

 general all that is required for a simple indicating system. Potentiom- 

 eters are hardly influenced by acceleration and vibration, but they 



1 See I. J. Hogan, IRE Convention, West Coast, Long Beach, August, 1952; 

 special reprint from Helipot Corporation, Newport Beach, Calif. 



2 For a discussion of the technical characteristics of potentiometer trans- 

 ducers, see L. A. Nettleton and F. E. Dole, Rev. Sci. Instr., 17, 356 (1946). 



