ELEMENTARY ELECTRICITY AND MAGNETISM. 



uniform. Let an electric wire be stretched between the poles 

 perpendicular to the plane of the paper. The composite field 

 due to the combined action of the magnet poles and the electric 

 wire is represented in Fig. 7, which is from a photograph. The 



/ } \ \ \ 

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v\ 



Fig. 8. 



black circle at the center of the figure is a section of the wire. 

 The trend of the lines of force near the wire in Fig. 7 is clearly 

 shown by the dotted lines in Fig. 8, which is from a drawing. 



Side push on an electric wire stretched across a uniform mag- 

 netic field, that is, a field which would be uniform but for the pres- 

 ence of the wire. The electric wire a, Fig. 8, is pushed sidewise 

 by the magnetic field as indicated by the arrow F. This force is 

 at right angles both to the magnetic field and to the wire, and it 

 may be ascribed to the tendency of the lines of force to shorten 

 themselves. 



11. Strength of current. Consider a straight electric wire 

 stretched across a uniform magnetic field of unit intensity, the 

 wire being at right angles to the field as described in the forego- 



