464 



FARM MOTORS 



lower. Moreover, if the tank A is not continuously sup- 

 plied with water the tank B will soon be filled to an equal 

 level; likewise if current is not supplied to the body 

 having the greater potential, the potential will become 

 the same in the two bodies. 



612. Volt or unit of potential difference. To measure 

 the amount of work required to transfer a charge from 

 one body of a high potential to one of a low potential 

 there must be a unit. This unit is called the volt in 

 honor of the great physicist Volta. It is roughly equal 

 to the P.D. between one of Volta's cells and the earth. 



CURRENT OR GALVANIC ELECTRICITY 



613. Current electricity. Electricity is an invisible 

 agent and is detected only by its effects or manifestations. 

 Current electricity is generally detected by its magnetic 

 effects. That is, near all currents of electricity there are 

 indications of magnetism, while in stationary or static 

 electricity there are none. 



614. Shape of magnetic 

 field about a current. If a 

 wire carrying a heavy cur- 

 rent of electricity be run 

 through a cardboard and 

 filings be sprinkled upon the 

 board they will form them- 

 selves into concentric rings 

 about the wire (Fig. 344). A 

 compass placed in this field 

 and at several positions will 

 show that the lines of force 

 are all in one direction. Re- 

 verse the current and the 

 j. 1G . 344 needle will also reverse. This 



