238 OUR PHYSICAL WORLD 



magnet. This turns the armature so that the free end c comes 

 into the position first occupied by a; it is made a south pole, and 

 thus the whole process is repeated. 



How the wires on the horseshoe magnet and the radiating 

 cores must be wound to produce the results described will be 

 clear if a simple law already learned is recalled (p. 207). If 

 an electric current is sent through a circular loop of wire placed 

 about a magnetic needle or small bar magnet that is free to swing 

 on its mid-point in a horizontal plane at right angles to that of 

 the loop, the needle or magnet is deflected. If a strong current 

 is used or if many turns of wire in a flat coil be used in place of 

 the single loop, the deflection is very marked, and the needle will 

 assume a position such that its long axis is perpendicular to the 

 plane of the coil. Furthermore, the direction of deflection will 

 be constant. If you imagine yourself swimming along the wire 

 in the direction in which the current is flowing, your chest toward 

 the needle, the north pole of the needle is always turned to the 

 left. Evidently the loop of wire is the equator of a magnetic 

 field whose south pole attracts the north pole of the needle 

 and coincides with it when the needle attains its maximum 

 deflection. 



If a current is sent through a spirally coiled wire whose turns 

 run in the same direction as the hands of a clock, or as the turns 

 of the thread on a right-handed screw, the spiral coil behaves as a 

 magnet, and magnetizes a soft-iron core placed within it. Apply- 

 ing the same swimming figure for determining its polarity, evi- 

 dently the end of the coil toward which the current is moving is 

 the south pole, while, if the turns of the wire are left-handed or 

 anticlockwise, the pole is a north pole. 



The electric motor is an exceedingly convenient device for 

 the application of power. It may be mounted directly on the 

 shaft that is to be turned, instead of being connected with it by 

 a crank shaft or by belting and pulleys, as is usually necessary 

 when a steam engine is used as the source of power. As it 

 occupies little space in proportion to the power developed, it 



