Sec. 1-2] MECHANICAL INPUT TRANSDUCERS 39 



The transducer in the form of Fig. ( I -2) 18 is subject to large hys- 

 teresis and drift and is severely influenced by transverse forces or 

 displacements. The most successful method to overcome these diffi- 

 culties consists of mounting the carbon electrodes, in the form of 

 rings, on an insulated core in a rigid frame, prestressing the carbon 

 column and using it only over a limited range of displacements. 1 The 

 hysteresis effect which can be as much as 20 per cent of the total 

 range is particularly high if the transducerisnewlyassembledbutcan 

 be reduced by mechanical aging, i.e., repeated application of load. 



The range of displacements for which the described form of the 

 transducer can be used varies from about 5 to 250 /u per interface; 

 displacements of a fraction of a Carbon composition 



micron can be detected ; the range / ~~n 



of force varies from about 100 — / /{ 7^~ — T — ~/i~$ / 



grams to 10 kg. As in any other / < *T j< -*J^Ba e 



transducer, the force range can '^ Electrodes^- 



be considerably extended (3,000 



i \2 j-u u ii r u Fig. (1-2)20. Carbon-composition strain 



kg r through the use ol median- l 



0/ & gauge. 



ical load cells, and the range of 



displacements can be increased by the insertion of an elastic member 

 (spring) between the moving element and the transducer. The resist- 

 ance at zero load depends upon the material and the surface treatment 

 of the carbon disks and can be between 0.5 and several hundred ohms. 

 The variation of resistance can be as high as 10 to 1 for a variation 

 from no-load to maximum load, but is in general much smaller. 



The mechanical resonance frequency of the carbon column can be 

 as high as 60,000 cps. 3 The mass moving with the column, such as 

 the frame or the electrodes, reduces, of course, the useful dynamic 

 range. Spark-plug-type pressure transducers have been built with a 

 natural frequency between 3,500 and 11,000 cps. 4 



A carbon-composition transducer is shown in Fig. (1-2)20. It con- 

 sists of an insulated carrier upon which is deposited a carbon com- 

 position such as finely divided graphite particles in a nonconducting 

 binder (shellac, plastic, resin, cellulose acetate) or simply graphite 

 suspended in alcohol. Two electrodes are applied. If the carrier is 



1 O. S. Peters and R. S. Johnston, Engineering, 116, 253 (1923), and Am. Soc. 

 Testing Materials, Proc, 23 (II), 592 (1923); O. S. Peters and B. McColtum, 

 Natl. Bur. Standards Tech. Paper 17 (247), 737 (1924); O. S. Peters, Am. Soc. 

 Testing Materials, Proc, 27 (II), 522 (1927). 



2 R. I. Martin and D. E. Caris, SAE Journal, 23, 87 (1928), and Automotive 

 Ind., 62, 230 and 592 (1930). 



3 W. Glamann and H. Triebnig, Forschung, 4, 137 (1933). 



4 Martin and Caris, loc. cit. 



