Polytechnic Association. 959 



It has been determined (Latimer Clark on Electric Measurement, 

 page 64) that one nautical mile, 2,029 yards, pure copper wire, weigh- 

 ing one pound, has, at 60° Faht., 1,155.5 ohms resistance. 

 1 flb.=7,000 Troy grains. 

 2,029 yds. : 7,000 grs. :: 10 yds. : 34.5 grs., and 

 2,029 yds. : 1,155.5 ohms:: 10 yds. : 5.695 ohms. 

 Therefore, 



10 yds. pure copper wire, weighing 34.5 grs., has 

 5.695 ohms resistance. 

 The resistance of a given length of wire is inversely proportional 

 to its weight ; hence, if ten yards wire weigh ten times as much, 345 

 grains, its resistance will be one-tenth =.57 ohms. 



If, on trial, we lind its resistance to be greater, say .67 ohms, its 

 conductivity is less than the pure copper in the inverse ratio of the 

 resistance ; that is : 



67 : 57:: 100 : 85; 

 or the metal has a conductivity of 85, the pure being taken at 100. 



Example. 

 Suppose ten yards pure copper weighs 173.4 grains, and has a 

 resistance of 1.2 ohms ; 



34.5 grs. : 5.695 ohms:: 173.4 grs. inversely : 1.133 ohms; i. e. 

 34.5x5.695 'l 



173.4 =1J33 - 

 For specific resistance : 



1.133 : 100:: 1.2: 105.91; 

 and for specific conductivity, the same proportion inversely : 

 1.133x100 .. . 

 1.2 = 94 ' 2 - 

 Therefore, taking both the resistance and the conductivity of pure 

 copper at 100, the specific resistance of the specimen tested is 105.9, 

 and its specific conductivity 94.2. 



41. For convenience, we may take the product of 34.5 grains x 5.695 

 ohms =196.4775, as a constant quantity, to be divided by the weight 

 in grains of any specimen of ten yards of copper wire. This will 

 give the resistance, in ohms, which the specimen would have if pure. 

 Dividing this (multiplied by 100) by the actual resistance, gives the 

 specific conductivity, or, dividing the actual resistance (multiplied by 

 100) by this, gives the specific resistance. 



