204 



GENERAL SCIENCE 



in a wire held directly over the compass, the AT pole of 

 the needle will move to the west (Figure 176). If the wire 

 is held under the compass, the N pole will move to the 



east. If the current 

 is reversed, the direc- 

 tion of the needle will 

 be reversed. 



Pass a wire carrying 

 a current through a 

 sheet of cardboard 

 which is held in a horizontal position (Figure 177). With 

 the aid of a compass test the magnetic field about the 

 wire. As the compass is moved it will be found that 

 the position of the needle changes in such a way as to 

 form a right angle with a line drawn from the 'wire to 

 the middle of the needle. The needle of the compass 



FIG. 175. An Electric Furnace. 



FIG. 176. Effect of Electric Cur- 

 rent on Magnetic Needle. 



FIG. 177. 



will be tangent to a circle drawn through that point 

 with the wire as the center of the circle. 



From these experiments it will readily be seen that a 

 wire carrying a current is surrounded by a magnetic field. 

 The magnetic lines of this field lie in circles about the 

 wire in such a way that if the wire is grasped with the 



