ELECTRICAL APPLIANCES AND MACHINES 259 



MAGNETIC LINES 



ABOUT A WIRE 



CARRYING A 



CURRENT 



Now determine whether the current is going up or 

 down through the wire, by examining how it is connected 

 with the poles of the cells. If we change the connection 

 to reverse the current we shall find that 

 the compass needles will also change their 

 direction. This means that the lines of 

 force have changed the direction in which 

 they are going around the wire. One way 

 of finding which way the lines of force 

 are going about a wire carrying a current 

 is by the right-hand rule. Grasp the wire 

 in the right hand with the thumb pointing 

 in the direction in which the current is go- 

 ing; the fingers then encircle the wire in the same direction 

 as the magnetic lines of force. 



Now let the wire carrying the current be bent into a 

 small coil of one turn and insert a 

 small compass when the coil is held 

 in a north-and-south position and 

 observe the direction in which the 

 needle points. If we reverse the di- 

 rection of the current by changing the connections on 

 the cells, the needle will also reverse its direction. In 

 both conditions the needle points in the direction in 

 which the magnetic lines of force are 

 passing through the coil. If these lines 

 of force are followed by moving the 

 compass in the direction in which the 

 needle points, we find that they come 

 out of one side of the coil, go around the 

 wire, and go into the other side of the coil. The side 

 into which the magnetic lines of force go is called the 

 south pole of the coil and the side from which they 



K 



THE RIGHT-HAND 

 RULE 



LINES OF FORCE 

 Around a single coil. 



