CP ARTS AND SCIENCES. 135 



Table VII. gives the capacities of wires from No. 0000 B. W. G. to 

 No. 18, when suspended above the ground at heights varying from 20 

 feet to 50 feet. 



Of course I have not measured the capacity of each size of wire at 

 each height, but I have chosen a large variety of sizes and heights, 

 and, having measured these, have calculated the remainder from these 

 by means of the formula 



a- kl 



in which 



C = capacity of the lines in microfarads. 



/ = length of line in miles. 



h = height above the earth in inches. 



d = diameter of lines in inches. 



k = the number .0496, i. e. the capacity of such a wire that 



= unity ; this number (k) being calculated from such 



log 4 A 



d 



wires as were actually measured. 



The capacity of any wire not given in the table may be calculated 

 from this formula. 



The capacities thus obtained apply to a single wire on a line of poles. 

 If there are other wires on the same poles, a correction must be added 

 depending upon the number of such wires, and their distance apart. 



For the ordinary case we meet with in telephony — i. e. a consider- 

 able number and placed about 18 inches apart — a sufficiently accurate 

 correction may be obtained by adding b0°f o . 



Table VIII. gives the capacities of different sizes of wire from No. 4 

 to No. 22 B. "W. G., when insulated with successive thicknesses of 

 gutta-percha from .01 inch to .25 inch, and combined into cables of 

 fifty conductors and enclosed in a metallic sheath. 



Table IX. is a similar table, in which India-rubber is used for in- 

 sulating: ; and Table X. one in which the conductors of the cable are 

 insulated with cotton so impregnated with paraffine as to be homoge- 

 neous. This table is applicable to the so-called Patterson cable if the 

 values be increased by 60%. 



These tables were prepared by measuring a wide variety of cable 

 conductors differing in size of conductor, thickness of insulating mate- 

 rial, and kind of insulating material, and calculating the remaining 

 values from the measured values by the formula 



