12 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



a. Reluctance-variation Gauges. Examples of thickness gauges 

 based upon reluctance variation are illustrated schematically in Fig. 

 (1-1)8. A core C of soft iron, usually laminated, carrying a coil is 

 brought in contact with the test object T . The reluctance of the core 



is small compared to that of the 



T s 



m 



L 



-c 



T. 



C 



W/////MA ^///^/>////// / . 



\a) 



(6) 



C 



,C 



WZ07,. 





(c) id) 



Fig. (1-1)8. Different types of reluc 

 tance-variation gauges. 



test ob j ect so that the reluctance of 

 the magnetic circuit depends pri- 

 marily upon that of the test sheet, 

 i.e., upon its thickness. Change of 

 the thickness leads to a change of 

 the coil inductance, Fig. (l-l)8a,&, 

 or c, or of the mutual inductance 

 between two coils, Fig. (l-l)8d. 



Empirical calibration for each 

 type of test material is usually 

 required, since the reluctance de- 

 pends upon the magnetic perme- 

 ability of the test material. For 

 this reason, the magnetic systems 

 are generally used for comparison 

 only, i.e., to detect the deviation 

 from a standard thickness, rather than to make absolute thickness 

 measurements. 



The transfer characteristic of these gauges is usually nonlinear. 

 The systems have been used for steel sheets of a thickness ranging 

 from less than 0.001 to about 0. 1 in., sometimes higher. The accuracy 

 is rarely better than 2 per cent; in most cases it is about 10 to 15 per 

 cent. The accuracy increases for thinner samples. 



For references see C. E. Richards, J. Electrodepositors' 

 Tech. Soc, 14, 101 (1938); W. H. Tait, ibid., 14, 108 

 (1938); W. E. Hoare and B. Chalmers, ibid., 14, 113 

 (1938). These references are cited by R. S. Bennett, J. 

 Sci. Instr., 26, 209 (1949). 



Several modifications of the reluctance vari- //////////////////■ 

 ation-gauge system have been described as follows : 



magnetic-shunt system. This system con- 

 sists of a permanent magnet or electromagnet A, 

 as shown in Fig. (1-1)9; it is brought in contact 

 with the ferromagnetic base B. A part of the flux 

 between the pole pieces passes through B, thus diminishing the flux 

 in the gap G. The magnetic field strength in the gap G can be meas- 

 ured with any one of the magnetic transducers described in 3-1. 



Fig. (1-1)9. Mag- 

 netic gauge; a part 

 of the magnetic flux 

 is shunted by the 

 test object B. 



