Polytechnic Association. 957 



i. e., multiply s by s and t by r, and add the products together ; sub- 

 tract from this amount t times s, and r times s ; subtract the square 

 root of the remainder from s, and the remainder will give the resist- 

 ance of the wire between the fault and the testing station. 



This test should, if practicable, be taken from both ends of the 

 wire, and greater accuracy is secured by taking the mean of several 

 observations. 



To Locate a Cross. 



36. The two wires in contact form a loop ; open both wires at the 

 nearest available point beyond the cross, and measure the resistance 

 of the loop. Half of this will be the resistance between the testing 

 station and the cross. The cross itself sometimes has considerable 

 resistance, which would make its true position somewhat nearer than 

 its apparent position. 



37. A cross may also be located by the method given (34), by put- 

 ting one wire as a ground, which will make an escape at the point 

 where the cross is situated, and which, of course, may be located in 

 the same manner as any other escape, by either of the methods above 

 given. 



Conductivity and Resistance. 



38. The conductivity of two conductors of same metal are directly 

 proportional to the areas of their transverse sections ; or, if of round 

 wire, they are directly proportional to the square of their diameters. 

 The resistance of the same wires «are inversely proportional to the 

 squares of their diameters. 



Specific Conductivity and Resistance. 



39. The relative specific resistance of two metals may be determined 

 by taking the resistance of a wire of each, of a given length and 

 diameter ; their resistances will denote their relative specific resist- 

 ances, or they may be computed from wires of different diameters 

 (their lengths being equal), by the following formula : 



D = diameter of standard wire. 

 R = resistance of standard wire. 

 d = diameter of wire to be tested. 

 r = resistance of wire to be tested. 

 IP : R :: d 2 inversely : r. 

 i. e.. 



