Sec. 1-2] MECHANICAL IS ITT TRANSDUCERS 37 



were obtained with films of Pd, Pt, Sb, Co, Au, Ni, and Te deposited 

 on a strip of anodized aluminum which was coated with silicone 

 resin. With decreasing thickness of the film, the sensitivity (gauge 

 factor) generally decreases first, i.e., becomes less than that of a 

 strain gauge in bulk (wire) form, and then rises above that of the bulk 

 material. Sensitivities 10 to 20 times higher than that of wire strain 

 gauges have been observed. Some of the film strain gauges exhibit 

 hysteresis, drift, and nonlinearity. 



Numerous modifications of the strain gauge have been proposed 

 to extend its application to higher temperature. Nichrome wire 

 mounted on ceramic-coated metal foil can withstand oxidation and 

 corrosion at temperatures up to 1000°C. The gauges can be mounted 

 to the test specimens by high-temperature cements, such as lead 

 silicate, silica oxide, or alumina oxide types, or by spot welding. 

 Glass- woven strain gauges, i.e., gauges made by braiding or weaving 

 glass-fiber-insulated wire into a ribbon, have also been used. The 

 application is limited; most low-melting glasses become conductive 

 at high temperature. Gauges for high temperature usually become 

 stable only after suitable aging, i.e., repetitive heating and cooling. 1 



The advantages of wire strain gauges are in their flexibility, their 

 small physical size, and their high stability and accuracy. Their 

 relatively small output impedance can be advantageous for tele- 

 metering applications. Disadvantages are the considerable force 

 required to elongate the gauge and the small output level (compared 

 to the capacitive or inductive displacement transducer) and, for the 

 bonded strain gauge, the fact that an appreciable time is required for 

 bonding the strain gauge to the specimen under test. The strain 

 gauge has been used in connection with many instrumentation 

 systems, such as load cells, pressure-measuring systems, torque 

 pickups, etc. 2 



For references, see W. B. Dobie and P. C. G. Isaac, "Electrical Resistance 

 Strain Gauges," The English Universities Press, Ltd., London, and the Mac- 

 millan Company, New York, 1948; C. C. Perry and H. R. Lissner, "Strain 

 Gage Primer," McGraw-Hill Book Company, Inc., New York, 1954; W. M. 

 Murray and P. K. Stein, "Strain Gage Techniques," Society for Experimental 

 Stress Analysis, Cambridge, Mass., 1958. 



d. Semiconducting Displacement Transducers. Two basically dif- 

 ferent types of semiconducting transducers exist, one where the mul- 

 tiple contact area between semiconducting particles (usually carbon) 



1 F. G. Tatnall, Summary Report on High Temperature Strain Gauge 

 Resistors, Contr. NONR-845(00), Office of Naval Research, Jan. 25, 1955. 



2 See A. C. Ruge, Natl. Bur. Standards Circ. 528, p. 93, 1954. 



