ABSTRACTS OF TECHNICAL ARTICLES 775 



with the intercity networks are discussed; and an analysis of some of the tele- 

 vision transmission problems is presented. A description is given of the physi- 

 cal and electrical characteristics of the various types of cable facilities, the 

 video amplifiers, and equalizers now employed; and an example of the 

 television transmission performance obtained is included. 



Significance of Composition of Contact Point in Rectifying Junctions on 

 Germanium. W. G. Pfann^ Letter to the editor. References. Phys. Rev., 

 V. 81, p. 882, Mar. 1, 1951. 



The Characteristics and Some Applications of Varistors* F. R. Stansel^ 

 Bibliography. I.R.E., Proc, v. 39, pp. 342-358, Apr., 1951. 



Abstract — Varistors, circuit elements whose resistance is a function of 

 the voltage applied, represent one important commercial application of 

 semiconductors. They may be divided into two classifications: nonsymmetri- 

 cal and symmetrical varistors. The first classification includes both metaUic 

 rectifiers such as copper oxide, selenium, and copper sulfide, and point 

 contact rectifiers such as silicon and germanium. The only commercial varis- 

 tor of the symmetrical class is the silicon carbide varistor, although a sym- 

 metrical characteristic may be obtained by connecting two nonsymmetrical 

 varistors in parallel with proper polarity. 



Each varistor has its volt-ampere characteristic and at each point on this 

 characteristic two different values of resistance may be defined, namely 

 the dc resistance, defined as the ratio of voltage to current, and the dynamic 

 or ac resistance, defined as the ratio of dE to dl. The former is important 

 in problems dealing with steady-state dc or large-signal applications, while 

 the latter is important when dealing with small applied signals. 



Because of the state of the art, varistors as manufactured commercially 

 are less uniform than many other circuit elements and required uniformity 

 is often obtained by special selection. Economical use of these elements 

 therefore requires the circuit engineer to recognize clearly which of the 

 several properties are important in his application and to specify special 

 selection for only those properties and to the extent necessary for his ap- 

 plication. 



Other properties of varistors which may be of importance are capacitance, 

 maximum inverse voltage, effect of temperature and frequency on any of 

 the other characteristics, long and short time stability, and noise. 



Of the many applications of varistors three are discussed which illustrate 

 how different properties may be determining factors in different applications. 

 In power rectifiers the limiting factors are those which may physically 

 damage the unit, energy dissipated within the varistor, and inverse voltage 



* A reprint of this article may be obtained on request. 

 1 B. T. L. 



