9 6 



ELEMENTS OF ELECTRICITY AND MAGNETISM. 



presence of the electric wire, would be a uniform field. The mag- 

 netic field between the flat-ended magnet poles in Fig. 48 is sensi- 

 bly uniform. Figure 61 shows the same field modified by the pres- 

 ence of a straight electric wire. The small black circle in Fig. 61 

 represents the section of the wire and the wire is perpendicular to 

 the plane of the figure. The magnetic field in Fig. 61 is due to 

 two distinct causes, namely, (#) the two flat-ended magnet poles, 

 and (&) the electric wire, and it may therefore be called a com- 

 posite field. If the field were due to the wire alone its lines of 



Fig. 62. 



force would be circles as shown in Fig. 57, and if the field were 

 due to the flat-ended poles alone its lines of force would be as 

 shown in Fig. 48. The trend of the lines of force in Fig. 61 in 

 the immediate neighborhood of the wire are more clearly shown 

 in Fig. 62 which is from a drawing. 



'Side push on an electric wire which is stretched across a uniform 

 magnetic field. The wire shown in Figs. 61 and 62 is pushed 

 sidewise by the magnetic field * as indicated by the arrow F in 



* Strictly, one should perhaps speak of the side force on the wire in Figs. 61 and 

 62 as due to the two magnet poles, because the two magnet poles constitute the actual 



