606 BELL SYSTEM TECHNICAL JOURXAL 



solutions are in general relatively good conductors, and that solutions 

 of substances in fat solvents are relatively good insulators, a much 

 closer analysis of the latter class is necessary for telephone purposes. 

 Among the electrical characteristics of importance are dielectric con- 

 stant, insulation resistance, a.c. conductivity, and dielectric strength. 

 Each of these characteristics varies over quite wide ranges, depending 

 upon certain conditions, of which temperature and frequency of alter- 

 nation of the electric current may be mentioned as most important. 

 In this work an endeavor is being made to determine, for example, 

 how the symmetry of the molecule affects the dielectric constant, and 

 to distinguish in dielectrics the contribution of energy loss made sever- 

 ally by the electron, the atom, and the molecule. For such a purpose 

 it is obviously necessary to deal with highly purified substances and to 

 begin with those of simple chemical structure. Many of these, such 

 as hexane, benzene, ethyl ether, and alcohol, are not expected to have 

 the slightest importance as practical insulators, but serve to show how 

 particular atomic groupings affect dielectric behavior. For this pur- 

 pose we have included the methyl halides in our study, making meas- 

 urements both in the pure state and in dilute solution, of dielectric con- 

 stant and a.c. conductivity at temperatures ranging from 100° C. to the 

 boiling point of the liquid. The frequency of the current has also been 

 varied from 1 to 100 kilocycles. 



Rubber and Gutta-Percha 



Parallel with this study of the theory of dielectric behavior, a number 

 of more immediately practical problems are being prosecuted, the ma- 

 jority of them under the direction of A. R. Kemp. One of these relates 

 to the use of rubber and gutta-percha in submarine insulation. The 

 latter is the classical material for this purpose, while rubber has been 

 regarded as an inferior substitute. The supremacy of gutta-percha is 

 due in part to its mechanical characteristic of thermoplasticity, which 

 permits it to be extruded as a continuous insulating layer about a con- 

 ductor, requiring nothing but the simple process of cooling to convert 

 it into a tough, firm sheath. An even more peculiar virtue of gutta- 

 percha is the stability of its electrical characteristics during prolonged 

 immersion in water. By patient experiments extending over several 

 years it has been demonstrated that the inferiority of rubber in this one 

 respect is due wholly to its non-hydrocarbon constituents. Methods 

 for the elimination of these foreign substances, notably water-soluble 

 salts, proteins, and quebrachitol, which could be applied without dam- 

 age to the hydrocarbon, have required extended study. 



