40 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



subjected to stress or flexed, the resistance between the electrodes 

 will vary. The variation of resistance occurs throughout the volume 

 of the composition layer (not on the surface, as above). 



Numerous formulas for the semiconducting mixture have been 

 described, varying in resistance from a few ohms to the megohm 

 range. For laboratory experiments it is sometimes sufficient to use 

 a pencil line or a line of india ink drawn on a piece of paper. 1 Conduc- 

 tive rubber (rubber rilled with finely divided graphite) also shows the 



400 



300 



200 

 o 



ftj 100 



ft* 



<1 







-100 



-200 



10 



20 30 40 

 Time, min 



50 60 



Fig. (1-2)21. Results of strain-cycle test on 

 three semiconductive strain gauges (from W. Ii. 

 Campbell, Natl. Bur. Standards Circ. 528, p. 

 138, Characteristics and Application of Resis- 

 tance Strain Gages, 1954; by permission). 



effect. Also ordinary carbon resistors are strain sensitive. 2 They can 

 be used as transducers with a reproducible characteristic in the tem- 

 perature range between —5 and — 30°C, but are damaged if subjected 

 to stress at higher temperature. 



A study of different compositions has been made by Campbell. 3 

 The mixtures are painted on a piece of tissue paper which is cemented 

 to a test specimen, similar to the arrangement of a wire strain gauge. 



1 S. L. de Bruin, Philips Tech. Rev., 5, 26 (1940). 



2 P. J. Rigden and H. J. H. Starks, J. Sci. Instr., 19, 120 (1942). 



3 W. R. Campbell, Natl. Bur. Standards Circ. 528, p. 131, 1954. 



