34 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



0.1 per cent of the full-scale output, apart from any long-term zero 

 drift. The dynamic response of the unbonded strain gauge depends 

 largely upon the mass of the moving parts to which the wires are 

 connected. The natural resonance frequency of the wires can be in 

 the range of several 10 kc, but practical strain gauges have usually 



Cover cemented 

 to base 



Electrodes 



Base- 



Resistance wire 

 cemented in position 



Fig. (1-2)15. Bonded strain gauge. 



a natural resonance frequency of not more than several hundred 

 cycles per second. 



bonded strain gauges. The bonded strain gauge (E. E. Sim- 

 mons, Jr., U.S. Pat. 2,292,549, 1942) consists of a grid of fine wire or 

 foil cemented to a paper support about 0.003 in. thick, as shown in 

 Figs. (1-2)15, (1-2)16, and (1-2)17. The wire is held in position by 



? ? 



Fig. (1-2)16. Resistance- 

 wire strain gauge. 



Fig. (1-2)17. Foil 

 strain gauge. 



nitrocellulose cement or phenol resin polymerized at high tempera- 

 ture. Two connecting leads are either soldered or welded to the fine 

 wire, and a piece of felt is frequently laid over the grid for thermal 

 protection. For application, the entire strain gauge is cemented to 

 the metal structure to be investigated, and the resistance variations 

 are measured. 



