Sec. 1-1] 



MECHANICAL INPUT TRANSDUCERS 







of the surface current i s through the meter M . The meter (galvanom- 

 eter: resistance R m ) measures, therefore, only the current passing 

 through the test object. A resistor R p is provided to protect the 

 meter in case of a flashover. If R is the resistance between the elec- 

 trodes A and C as indicated by the meter [R = E/I - (R p + Rm)], 

 the thickness of the test object can be found from 



Ufa + r 2 ) 2 7r 



t = 



4p 



The electrodes should be in intimate contact with the material to be 

 investigated. Mercury makes good contact but is difficult to handle. 

 Electrodes of platinum, gold, or silver, which are deposited chemi- 

 cally by cathode sputtering or by means of a metal sprayer, give good 



Fig. (1-1)4. Guard-ring arrangement 

 for the separation of surface and volume 

 resistance. 



Fig. (1-1)5. Contact square for 

 thickness measurement of a plate 

 when only one side is accessible. 



results. The use of colloidal graphite ("Dag") has been suggested. 

 For many purposes fine tin or aluminum foil held in place by a thin 

 layer of paraffin oil or petroleum jelly is satisfactory. 



b. Contact on One Side. The following method permits the deter- 

 mination of the thickness of a metal plate if only one side is acces- 

 sible. 1 The method is based upon the difference of the current flow 

 pattern in a thin sheet (two-dimensional flow) and in a thick sheet 

 (three-dimensional flow). 



Current is passed through the plate under investigation between 

 two contacts, A and B, Fig. (1-1)5, and the potential difference 

 between two other contacts, C and D, is measured. In a thin sheet 

 the potential difference for constant current depends upon the geom- 

 etry but not upon the actual spacing of the electrodes; in the thick 

 sheet, the potential difference for constant current is inversely pro- 

 portional to the linear dimensions. In a plate of finite thickness t the 

 conditions are approximately those of a two-dimensional flow if the 

 electrode distance s is large, i.e., for sjt j> 1 ; in this case the potential 

 difference for similar electrode arrangements is independent of the 

 electrode spacing. However, if the electrode distance is small, the 



1 A. G. Warren, J. Inst. Elec. Engrs. {London), 84, 505 (1939). 



