2 PROCEEDINGS OF THE AMERICAN ACADEMY 



It is etident, therefore, Miat in the coustruction of a telephone line 

 it is desirable to reduce both the resistance and the capacity to a 

 minimum. 



In a pole line, since the wire is suspended high above the earth, 

 the capacity is always small, and the resistance is the factor that we 

 must try to keep down. 



In cable lines, however, where the conductor is necessarily brought 

 near to other conductors, or a metal shield, or the earth, the capacity 

 becomes quite an im^iortant factor to be respected. 



In lines made up, as is most generally the case, of a comparatively 

 short section of cable and a larger section of iron pole wire, the ca- 

 pacity of the cable becomes pre-eminently the factor to be respected ; 

 for, since the limit of conversation is here determined by the product of 

 the capacity of the cable and the resistance of the whole line, a small 

 percentage of saving in the capacity of the cable gives an enormous 

 gain in the readiness with which conversation may be carried on over 

 the line. 



It becomes of vital importance, therefore, to choose an insulating 

 material for telephone cables of low specific inductive capacity. 



We have accordingly measured the electrostatic capacity of a con- 

 siderable number of substances used for insulating wires in cables, 

 and, since the specific inductive capacity for the same insulator is very 

 different for telephone currents from what it is for telegraph currents, 

 because the charge and discharge take place so much more frequently, 

 we have adopted a method and apparatus in which currents of tele- 

 phonic frequency are used. 



This apparatus is a Gordon induction balance, (see Gordon's Elec- 

 tricity and Magnetism, p. 110,) in which the substances were charged 

 and discharged by currents from an induction coil, and the balance 

 was observed with a telephone instead of an electrometer. 



With this apparatus, we measured the specific inductive capacity of 

 a considerable number of insulating materials, and each when sub- 

 jected to various rates of charge. 



We found that the capacity varied very materially with the rate of 

 charge, some substances increasing and others decreasing, but no 

 general rule was evolved. 



We found that measurements of the capacities of telephone cables 

 made by the ordinary galvanometric measurements gave no indications 

 of the comparative merits of these cables for telephonic work. 



Finally, we made an accurate series of measurements of the spe- 

 cific inductive capacity of the following substances, when submitted to 



