CHEMISTRY IN THE TELEPHONE INDUSTRY 611 



in that an insulator of maximum dielectric constant is required, where- 

 as in most electrical apparatus a minimum dielectric constant is sought. 

 For the sake of economy the usual telephone condenser is made of al- 

 ternate strips of paper and tinfoil which are wound up into a compact 

 roll, and after drying is impregnated with some form of waxy material 

 to bring the capacity to a maximum and prevent subsequent variations 

 with changes of atmospheric humidity. The complexity of the effects 

 of the choice of the impregnating material upon the electrical charac- 

 teristics of the condenser is most surprising. It might be supposed 

 that condensers of high insulation resistance would also have a high 

 breakdown strength, but this is by no means always the case. It has be- 

 come evident that the nature of the interface between the individual 

 fibers of the paper and the surrounding waxy material is of vital im- 

 portance. 



Phenol Plastics 



A class of materials very widely used in the electrical industry for 

 insulating purposes is well known under the general term of "phenol 

 plastics." Cellulose acetate is another insulating material of excellent 

 electrical characteristics and is superior to the usual fibrous materials 

 owing to its lower water absorption. Although the uses of such mate- 

 rials, both in massive form and as impregnants for fibrous insulation, 

 are very extensive in the telephone field, little chemical work has been 

 done in the Laboratories on them, partly because they are the products 

 of a rather highly developed industry which has conducted a great deal 

 of investigation for us. For certain types of uses, however, the phenol 

 plastics have required some special chemical study from the standpoint 

 of their stability. Being in the nature of condensation products formed 

 by the elimination of water, the phenol plastics are more or less subject to 

 the reversion of the reaction by which they were formed with the produc- 

 tion of free phenol and ammonia. For certain uses the presence of these 

 uncondensed constituents or hydrolytic products, as the case may be, 

 is objectionable and has required a special investigation of means of 

 controlling their presence. An important improvement and economy 

 has been effected in certain textile-insulated wires by applying a cellu- 

 lose acetate lacquer to the exterior so as to partially impregnate the 

 textile. The film of cellulose acetate contributes a continuous smooth 

 surface and a measure of resistance to atmospheric humidity variations. 



Conductivity of Copper 



Passing from insulators, one naturally thinks of conducting materials, 

 of which of course copper is first in importance. It is a well-known and 

 thoroughly tried principle that the metallic elements in the pure state 



