136 PROCEEDINGS OF THE AMERICAN ACADEMY 



NLI NLI 



C = 



log 2s" log 2 d -f 4 T 



d d 



in which 



C = capacity of the conductor in microfarads. 



s = average distance between centres of adjacent conductors. 



d = diameter of conductor. 



2 1 = thickness of insulating coating of any one conductor. 



L = length of conductor in miles. 



I = specific inductive capacity of insulating material used. 



N= the number 0.0387, i. e. the capacity of one mile of such a wire 



that = unity. 



The number N is calculated from values reached by experiments 

 on a large number of conductors. 



The capacity of any cable conductor not given in these tables, and 

 insulated with any material whose specific inductive capacity is known, 

 may be calculated from this general rule. 



Having thus determined a general rule for the construction of any 

 telephone circuit, and having provided in the foregoing tables data by 

 which the rule may be applied, I will give an illustration, by apply- 

 ing it to the particular case above cited, namely a telephone line 

 between two cities one hundred miles apart, entering each city by 

 underground cables two miles in length. 



Let us further suppose that the subscriber is in each case connected 

 to the central office by a mile of undergronnd cable, and that at each 

 central office, there is a multiple switchboard, any connection through 

 which has a resistance of 25 ohms, and a static capacity of .10 micro- 

 farad. 



It is desired to use Blake transmitters. 



Resistance. 



Ohms. 



Line of No. 13 copper on 30 ft. poles, 6.048 X 90 . . . 571 

 Cable of No. 18 insulated with kerite to No. 10, 22.7 X 6 . 136 

 Switchboard 25 



Total resistance 732 



