Relations between Electrical Measurements. 519 



50. Conducting -Power } Specific Resistance, and Specific Con- 

 ducting -Power. 



Conducting '-Power, or Conductivity. — These expressions are 

 employed to signify the reciprocal of the resistance of any con- 

 ductor. Thus, if the resistance of a wire be expressed by the 

 number 2, its conducting-power will be 0*5. 



Specific Resistance referred to Unit of Mass. — The specific re- 

 sistance of a material at a given temperature may be denned as 

 the resistance of the unit mass formed into a conductor of unit 

 length and of uniform section. Thus the specific resistance of a 

 metal in the metrical system is the resistance of a wire of that 

 metal, one metre long, and weighing one gramme. 



The Specific Conducting- Power of a material is the reciprocal 

 of its specific resistance. 



Specific resistance, referred to unit of volume, is the resistance 

 opposed by the unit cube of the material to the passage of elec- 

 tricity between two opposed faces. It may easily be deduced 

 from the specific resistance referred to unit of mass, when the 

 specific gravity of the material is known. 



Specific conducting-power may also be referred to unit of 

 volume. It is of course the reciprocal of the specific resistance 

 referred to the same unit. 



It is somewhat more convenient to refer to the unit of mass 

 with long uniform conductors, such as metal wires, of which the 

 size is frequently and easily measured by the weight per foot or 

 metre ; and it is, on the other hand, more convenient to refer to 

 the unit of volume bodies, such as gutta percha, glass, &c, 

 which do not generally occur as conducting-rods of uniform sec- 

 tion, while theird imensions can always be measured with at least 

 as much accuracy as their weights. 



51. Specific Inductive Capacity. — Faraday* discovered that 

 the capacity of a conductor does not depend simply on its dimen- 

 sions or on its position relatively to other conductors, but is in- 

 fluenced in amount by the nature of the insulator or dielectric 

 separating it from them. The laws of induction are assumed to 

 be the same in all insulating materials, although the amount be 

 different. The name " inductive capacity " is given to that qua- 

 lity of an insulator in virtue of which it affects the capacity of 

 the conductor it surrounds, and this quality is measured by refer- 

 ence to air, which is assumed to possess the unit inductive capacity. 

 The specific inductive capacity of a material is therefore equal 

 to the quotient of the capacity of any conductor insulated by 

 that material from the surrounding conductors, divided by the 

 capacity of the same conductor in the same position separated 



* Experimental Researches, series xi. 



