Sec. 4-5] ELECTRICAL TRANSDUCERS 22U 



Usually a voltage rather than a current output is required and can be 

 obtained by the insertion of a resistor R, as shown in Fig. (4-4)3. If 

 R < 1/C, then the output voltage is 



e = — E c RcoC - cos cot 



The method can be transformed into a null method by inserting in 

 the circuit an adjustable voltage E p derived from a potentiometer. 

 If E p is equal to E c and of opposite polarity, the output voltage e 

 disappears. Changes of potential of the order of 10 ^V can be 

 measured. 



The method is very sensitive. The presence of polar vapors (such 

 as ethyl alcohol) in the laboratory air of a concentration by weight 

 of 1 part in 10 7 can change the surface potential of a stainless-steel 

 plate by voltages in the millivolt range. 1 A gold-plated reference 

 electrode covered with a layer of calcium palmitate about 20 mole- 

 cules thick is recommended. 



For further references, see A. A. Frost, loc. cit.% S. Rosenfield and W. M. 

 Hoskins, Rev. Sci. Instr., 16, 343 (1945); W. E. Meyerhof and P. H. Miller, 

 Jr., using a single forward movement of one electrode, Rev. Sci. Instr., 17, 15 

 (1946); H. H. Kolm, apparatus with rotary movement, Rev. Sci. Instr., 27, 

 1046 (1956); H. G. Yamins and W. A. Zisman, J. Chem. Phijs., 1, 656 (1933). 



4-43. Surface Charges on Insulators 



The (usually irregularly distributed) electrostatic charges on the 

 surface of an insulator can be detected and measured by the use of 

 a moving electrode. An instrument of this type is described by 

 Devins and Reynolds. 2 It consists essentially of a fine wire, its end 

 surface (area, 0.05 mm 2 ) scanning in a rotatory motion the surface 

 to be examined at a distance from the surface of about 1 mm. A 

 charge on the insulator surface changes the potential of the wire by 

 electrostatic induction. The resolution is of the order of 1 mm. 



4-5. Electric Fields 



In general, the local electric field strength F = dEfds in a d-c field 

 cannot be converted directly into an electric signal. Indirect-acting 

 systems furnishing an electric output in response to electric d-c field 

 strengths are the following: 



1 G. Phillips, J. Sci. Instr., 28, 342 (1951). 



2 J. C. Devins and S. I. Reynolds, Rev. Sci. Instr., 28, 11 (1957). 



