132 ELECTRICITY 



high temperature to one whose temperature is lower, so 

 electricity may pass from a point having high potential 

 to a point of lower potential. 



The potential of two points is described as high and 

 low, or positive (+) and negative ( ) ; and this means 

 that their electrical condition is such that electricity 

 would tend to pass from one to the other. The one 

 from which electricity would pass is called positive and 

 the one to which it would pass is said to be negative. 

 This means only that the potential of the first is higher 

 than that of the second, without regard to any fixed 

 standard. 



156. Electro-Motive Force (E.M.F.) Different sub- 

 stances vary in the ease with which they carry elec- 

 tricity, but even the best of conductors offer some 

 resistance to the electric current passing through them. 

 To overcome this resistance a certain sort of electri- 

 cal " pressure " is required ; and this is furnished by a 

 difference in potential between two points in the con- 

 ductor. If one point has a high potential and another 

 point has a lower potential, a current may be caused to 

 flow from the first point toward the second. This may 

 perhaps be better understood if we compare it with the 

 flow of heat from a body of high temperature to another 

 of lower temperature. 



The difference in potential between two points in a 

 conductor, which causes the current to flow and to 

 overcome resistance, is called the electro-motive force of 

 the current. The greater the difference between the 

 potentials of two points, the greater is the electro-motive 



