492 Curtis: properties of solid dielectrics 



tive normal induction data in which the error in the magnetizing 

 force is not greater than 5 per cent. 



Uncorrected hysteresis data for hard steels show values of 

 the residual induction that are too small by an error which may 

 be as much as 10 per cent, while the coercive forces are too 

 large by an error which may be as much as 40 per cent. 



The chief value of such an instrument is for comparative 

 work in which it is desired to determine the degree of uniformity 

 of material or the relative values of similar materials. 



PHYSICS. — The insulating properties of solid dielectrics^ Har- 

 vey L. Curtis, Bureau of Standards. 



Two properties of dielectrics are considered: (1) the volume 

 resistivity and (2) the surface resistivity. The volume resistiv- 

 ity of a material is the resistance in ohms between two opposite 

 faces of a centimeter cube. The surface resistivity is defined 

 as the resistance between two opposite edges of a centimeter 

 square of the surface film which is deposited upon the material. 



In measuring the volume resistivity, mercury electrodes were 

 employed in order to make good contact, and a guard-ring was 

 used to prevent any errors on account of surface leakage. The 

 effects of the temperature of the specimen, of the humidity of 

 the smTounding air, and of the magnitude and length of appli- 

 cation of the applied voltage were studied. The only one of 

 these which, for ordinary laboratory conditions, affects the order 

 of magiiitude of the results is the length of application of the 

 voltage, and this is of importance only if the volume resistivity 

 is greater than 10^^. Values of the volume resistivity are given 

 in Table I. 



Since the surface resistivity depends upon the surface film 

 any condition which will affect this film will affect the surface 

 resistivity. Upon the surface of all insulators except the waxy 

 materials a film of moisture collects from the surrounding air. 

 The thickness and conductivity of this film depends upon the 

 material of which the insulator is composed and upon the relative 



1 Detailed paper to appear in the Bureau of Standards Bulletin. 



