4 



Electrical Transducers 



Electrical transducers convert certain electrical quantities into elec- 

 tric signals. An electric signal, as defined in theintroduction, iseither 

 a voltage, a current, an impedance, or a variation or a time function 

 of these quantities. The input quantity applied to the transducer is 

 of an electrical nature but does not have the character of an electric 

 signal; it may be a free electric charge, such as free electrons or ions 

 (4-1), or a space charge (4-2), or it may be a space potential (4-3), a 

 surface charge or surface potential (4-4), or an electric field strength 

 (4-5). 



4-1. Transducers Responding to Free Charges 



The two most frequently used transducer systems for the measure- 

 ment of free electrons and ions with a kinetic energy larger than 

 thermal energy are the charged plate (4-1 la) and the Faraday cage 

 (4-1 1&). Positive and negative charges of thermal energy in atmos- 

 pheric air can be measured with the Gerdien ion counter (4- 12a). A 

 system for the measurement of the excess charges in air (positive 

 minus negative charges) is described under 4-126. A method for the 

 measurement of moving ion clouds is described in 1-68. 



Electrons, positive ions, and alpha particles can also be measured 

 with electron multipliers, similar to the photomultipliers described 

 in 5-1 lc. The efficiency of the electron multiplier for incident elec- 

 trons of an energy of 500 eV is almost 100 per cent, but decreases for 

 lower as well as for higher energy. 1 



1 J. S. Allen, Rev. Sci. Instr., 18, 739 (1947), and Z. Bay, Rev. Sci. Instr., 12, 

 127 (1941); literature references in both papers. 



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