38 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



varies when the distance between these particles is changed, and one 

 where the resistance of a continuous film of a p- or w-type semi- 

 conductor, such as germanium, varies with stress like that of a thin 

 metal film strain gauge. Very little information is, as yet, available 

 on the latter-type transducer, while a considerable amount — mostly 

 technical information — has been published on the carbon- or carbon 

 composition-type transducer. 



One form of carbon transducer is shown in Fig. (1-2) 18. It consists 

 of two or more (up to 60) carbon disks mounted between a fixed and 

 a movable electrode. When a force 

 F is applied, the carbon disks 

 move together by an amount d, 

 and the resistance R decreases. 



The transfer function, R —f(F) 

 or R = f(d), is approximately j= 

 hyperbolic, as shown in Fig. (1-2) °40 

 19. Several workers have tried to 



80 



Mobile electrode 

 Carbon disks <^j. 



Fixed electrode 



20 



IT 



d 



1 



R 



4 

 kg,F 



i li 1 1 1 I 



100 



I I I ! L.J 1_^_ _L 



150 



200 



M 



Fig. (1-2)18. Semiconductive displace- 

 ment transducer. 



Fig. (1-2)19. Characteristic of a semi- 

 conductive (carbon disk) displacement 

 transducer. 



represent this relationship by empirical equations, but none of these 

 equations is valid over an extended range. Attempts to obtain linear 

 transfer functions have been made ; the most successful method con- 

 sists of an arrangement of two transducers in adjacent arms of a 

 resistance bridge, in which the pressure in one column is increased 

 and that in the other decreased. 



The resistivity of the carbon disks should be between 10 -1 and 10 3 

 ohm-cm; a resistivity of the order of 2 ohm-cm has been recom- 

 mended. 1 The material should be hard and should have a large 

 modulus of elasticity. The carbon disks should be thin, since the 

 resistance variation occurs only at their surface, not within their 

 volume. 



1 W. Glamann, Arch. tech. Messen, V 132-12, March, 1936. 



