32 



Mr Searle, Notes on a Vibration Magnetometer 



T 5 g inch ("79 cm.) in diameter, their centres being 10*41 cm. apart, 

 was placed in the "end on" position near a tangent magnetometer. 

 Denoting the distances (in centimetres) of the centres of the two 

 balls from the centre of the magnetometer by r x and r 2 , the results 

 were as follows : 



The last line gives the values of m/H E , calculated on the 

 assumption that the magnet acts as if it had two poles of strength 

 m, situated at the centres of the balls. The close agreement 

 between the numbers in the last line shews that the assumption 

 is very nearly exact. 



Magnets such as this, having a pole-strength of from 10 to 

 15C.G.S. units, are very useful for experiments in which lines of 

 force are traced out on a drawing-board by the aid of a search 

 compass. 



An alternative method of testing these magnets is to fit a 

 small coil to the rod and to slide it from point to point along the 

 rod, and to note the "throws " of a ballistic galvanometer connected 

 in series with the coil. Such a test on a magnet made of a 

 hardened steel wire "19 cm. in diameter and about 60 cm. in 

 length, fitted with mild steel spheres 112 cm. in diameter, shewed 

 that nine-tenths of the magnetic induction entered and left the 

 magnet by the spheres, and that two-thirds of the remaining tenth 

 entered and left the wire within 2 cm. of the spheres. A further 

 experiment, made by jerking a small coil away from various parts 

 of the surface of one of the balls, shewed that the flow of induction 

 from the surface was uniform to about 25 per cent. 1 



§ 8. By using one of the vibration magnetometers described 

 in | 2, in conjunction with a Robison magnet, students are able to 

 test the law of the inverse square by Coulomb's vibration method 

 with some accuracy. 



The experiment is arranged in the following manner: — On a 



1 A qualitative test of the ball-ended magnets was demonstrated to the Society 

 by optically projecting on a screen the forms taken up by iron filings near a plain 

 bar magnet and near a magnet formed of an exactly similar bar fitted with balls. 

 The lines of force indicated very clearly that the Bobison magnet had by far the 

 better "poles." 



