Mr Searle, Notes on a Vibration Magnetometer etc. 27 



Notes on a Vibration Magnetometer and on the Ball-ended 

 Magnets of Robison. By G. F. C. Searle, M.A., Peterhouse, 

 University Lecturer in Physics, and Demonstrator in Experimental 

 Physics. 



[Read 10 November 1902.] 



§ 1. The comparison of the horizontal components of magnetic 

 fields by the method of vibrations forms one of the experiments 

 in most elementary courses of practical physics, and presents no 

 difficulty when each of the magnetic fields is so nearly uniform 

 that a vibrating magnet several centimetres in length may be 

 used. But when the fields are far from being uniform the vibrating 

 'magnet must be quite short. The magnet must also be slender, 

 for unless its length be at least 10 times its diameter, the magnetic 

 moment varies appreciably when the field varies, even though the 

 maximum field is not greater than 3 or 4 times the earth's 

 horizontal force. 



But a simple magnet 1*50 cm. long and '15 cm. in diameter has 

 some disadvantages. Its small length makes it difficult to observe 

 small arcs of vibration, though with care some accuracy in timing 

 can be reached. Thus for such a magnet vibrating in the earth's 

 magnetic field I found for the time of 50 vibrations 71-1, 71*4, 71*0, 

 7l"5, 7T1, mean 71*22 sees., the initial arc being 10° on either side 

 of the zero. Further, when such a magnet is suspended in a non- 

 uniform field, it is drawn aside so that the suspending fibre is no 

 longer vertical. But the resultant horizontal force exerted on the 

 magnet changes as the angle between the magnet and the direction of 

 the horizontal magnetic force varies, and hence, if the time of vibra- 

 tion of the magnet about a vertical axis be comparable with the 

 time of the pendulous vibration of the magnet about the point from 

 which the suspending fibre is hung, the vibration about the vertical 

 axis will give rise to a pendulous vibration. In fact it is easy to 

 arrange matters so that the motion of the magnet is alternately a 

 pure vibration about a vertical axis and a pure pendulous oscilla- 

 tion. But, however interesting this motion may be to a spectator, 

 it is useless as a means of determining the strength of the magnetic 

 field. 



§ 2. In arranging the experiments for the examination in 

 Practical Physics in Part I. of the Natural Sciences Tripos, 1899, 

 I endeavoured to provide the Candidates with vibration magneto- 



