36 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



in expansion of the gauge wire and of the mounting, so that a strain 

 is caused in the wire, which is superimposed to that caused by the 

 actual displacement under measurement. Therefore, the gauges and 

 the mechanical equipment under investigation must be kept at a 

 constant temperature, or temperature compensation methods must 

 be applied. The most common method of temperature compensation 

 is to use a second strain gauge (dummy gauge) ; both gauges are 

 exposed to the same temperature, but only one is exposed to the 

 displacement under investigation. Temperature compensation is of 

 minor importance in dynamic investigations. Thermoelectric effects 

 arising at the lead connections may also cause errors. 



Humidity entering the gauge system is likely to cause disturbances 

 by creating insulation defects and corrosion of the wire. A protective 

 coating is sometimes required. 1 



Magnetorestrictive effects in strain gauges consisting of ferromag- 

 netic wires have been observed by Vigness. 2 The effect, which can 

 cause disturbing voltages in the order of 2 mV, is not observed in new 

 strain gauges but appears after the gauges have been "conditioned" 

 by the application of a current and exposed to repeated strain 

 impacts. 



Gauges cemented with nitrocellulose cement can be used without 

 appreciable error at temperatures which range as high as 60 to 80°C; 

 gauges made from cured synthetic resin can be used up to 200°C. 

 Good adherence of the strain gauge to the specimen under investi- 

 gation is important. Nitrocellulose cement dries in air at room tem- 

 perature (minimum time for drying, 3 to 4 hr; drying time for better 

 results, 10 hr) and can be used at temperatures below 80°C. Phenol 

 resin requires polymerization at elevated temperature (95°C for 12 

 hr) but is more stable and can be used up to 250°C. Air bubbles and 

 cavities between the specimen and the strain gauge are likely to 

 cause considerable errors. Errors may arise through hysteresis and 

 creep. Commercial gauges are available in lengths from J to 6 in. 

 and in widths from ^ to about J in. 



Strain gauges made by evaporating metals on an insulating sub- 

 strate have been investigated by Campbell. 3 Satisfactory results 



1 Details on cementing and waterproofing are described by R. G. Boiten, 

 Characteristics and Applications of Resistance Strain Gauges, Natl. Bur. 

 Standards Circ. 528, 1954. 



2 I. Vigness, Rev. Set. Instr., 27, 1012 (1956); see also J. M. Krafft, Proc.Soc. 

 Exptl. Stress Anal., 12 (2), 173 (1955), and A. Meitzler, Rev. Sci. Instr., 27, 56 

 (1956). 



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

 A. Krimsky and R. L.Parker, Natl. Bur. Standards Rept. 5139, February, 1957. 



