138 ON MAGNETISM. 



Upon carrying the mass of iron round the compass 

 in the manner described, the phenomena are these : 

 When the central point of the mass (if symmetrically 

 shaped), or a certain central point (in general) is on the 

 N. or S. or E. or W. side of the compass-center, it pro- 

 duces no disturbance in the compass-needle. When 

 the direction of that central point is between N. and E., 

 it turns the N. end of the needle to the E. : when be- 

 tween E. and S., it turns it to the W. : when the central 

 point of the mass is between S. and W., it turns the 

 N. end of the needle to the E. : when between W. and 

 N., it turns the N. end to the W. On comparing these 

 with the deviations produced by a magnet which is 

 carried round the compass in the same manner, as de- 

 scribed in Article 55, it is seen that there is a striking 

 difference ; in the case of a complete revolution of the 

 magnet, the needle is made to deviate once to the right 

 and once to the left ; but in the case of a complete re- 

 volution of the soft iron, the needle deviates twice to 

 the right and twice to the left. If the azimuth of the 

 disturbing mass, as viewed from the center of the com- 

 pass, and measured from N. towards E. be called 0, the 

 amount of deviation produced in the needle from N. 

 towards E. is exactly or approximately proportional to 

 sin 20 : vanishing when is 0, 90, 180, 270, and be- 

 coming negative when is > 90 < 180, or > 270 < 360. 

 The law of disturbance may be represented (for memory 

 only) by this rule : the mass attracts that pole of the 

 needle which is nearest to it. 



