IMPORTANT DEFINITIONS. 7 



The resistance of a conductor represents the magnitude of 

 the obstacles offered by its material particles to the free 

 passage of the fluid : it is the inverse of the conductivity. This 

 resistance depends, therefore, on the nature of the conductor 

 and on its dimensions. Ohm has shown that this resistance 

 is, for the same kind of conductors, inversely proportional to 

 the section of the conductor , that is to say, to the surface ob- 

 tained by cutting it perpendicularly to its length, and he also 

 found that the resistance was proportional to the length, and 

 inversely proportional to the. conductivity. In conductors of 

 indefinite mass like the earth, the resistance becomes inde- 

 pendent of the distance between the points where the circuit com- 

 municates with that mass, and depends only on its mean 

 conductivity and on the surface of the plates which establish 

 the communication ; it may therefore be considered as in- 

 versely proportional to the square root of that surface. 



As a circuit is usually composed of several different kinds 

 of conductors, which therefore offer different resistances for 

 the same dimensions, it becomes important, in order to esti- 

 mate the total resistance, to reduce them all to terms of one 

 and the same unit of resistance,* and when that is done, the 

 resistance of the circuit is termed the reduced resistance. The 

 laws established by Ohm relate only to reduced resistances- 

 It ought always to be borne in mind that however different 

 may be the several parts of a circuit, the electric intensity of 

 the current which traverses it is the same at every point, but in 

 each of its parts the tensions are different.f 



* This unit of resistance has long been discussed, and at the present day 

 electricians are still at variance as regards the one that should be adopted ; 

 in general, however, the British Association unit is employed, to which the 

 name of Ohm is given. This unit represents about 100 metres of telegraph 

 wire of 4 millimetres diameter. We shall refer to these units again. 



f According to the English formula of Latimer Clark, the following are the 

 values of the resistances of the most commonly used metals, for i metre of 

 length and i millimetre of diameter : 



Resistance in Conducting 

 Ohms. Power. 



Annealed Silver 0*01937 



Drawn Silver 0*02103 ... loc'oo 



