ELECTRICAL APPLIANCES AND MACHINES 251 



of the greatest pressure, or the difference in pressure is 

 the cause of the flow. 



The electric current is caused by the difference in 

 electrical pressure at the two ends of a conductor. This 

 difference of electrical pressure is called potential. (Po- 

 tential means to be able, or to have power.) Electricity 

 flows from high potential to low potential. High poten- 

 tial is positive and low potential is negative, so another 

 way to designate the direction of flow of electricity is 

 from positive to negative. These, however, are terms 

 used for convenience, as nothing definite is known about 

 which way electricity flows. 



174. Electrical Units. The difference in potential 

 at any two points on a conductor is the pressure that 

 makes the current flow from one point to the other. 

 All conductors offer some resistance, and hence this 

 pressure is necessary. This pressure is also called elec- 

 tromotive force (E. M. F.). The unit of electromotive 

 force is the volt, named after Volta, an Italian scientist. 

 Electric lamps in the house are usually lighted with a 

 current of no volts. Electric street cars use from 500 

 to 600 volts. 



The strength of current is measured by a unit called an 

 ampere, named after a French physicist. A unit current 

 is a current of one ampere. 



All conductors offer resistance to an electric current, 

 and the unit for measuring this resistance is the ohm. 

 We may now define volt as that potential difference or 

 E. M. F. which will drive a current of one ampere through 

 a resistance of one ohm. These units will be more 

 clearly understood when we work with electric cells and 

 machines. However, the relation that these three units 

 bear to one another can be partially understood from the 



