Determinations of Magnetic Susceptibility. 



405 



with, "which the results can be obtained ; while its disadvantage is 

 revealed in the fact that it does not show either slow changes of mag- 

 netisation or the distribution of magnetism. 



The following results of the experiments which have been made at 

 the Physical Laboratory of the University of Glasgow, are given in abso- 

 lute measure in which a centimetre, a gramme, and a second are taken 

 as the units of length, mass, and time respectively, and were arrived 

 at by means of the direct magnetometric method given to me by Sir 

 William Thomson (who described and explained the method at the 

 recent meeting of the British Association at Southampton in the 

 Section A), as founded upon a method originated by Coulomb and 

 discussed mathematically by Green. This method possesses some 

 important advantages over the electromagnetic method ; for instance, 

 it shows at any moment any change of magnetisation of the body 

 experimented on (which is of great practical utility in investigations 

 of this kind) ; it affords an excellent means of illustrating the distri- 

 bution of magnetism in the body, and it enables us to experiment 

 upon a long thin bar, subjecting it to different strengths of magnetising 

 forces, and to various amounts of longitudinal stress, and at the same 

 time to determine in absolute measure, the magnetisation and magnetic 

 susceptibility of the bar under these varied circumstances, which is an 

 original feature of the investigations I am going to describe. These 

 advantages, however, do not exist without disadvantages. That the 

 execution of careful investigations involves a considerable amount of 

 time, is a serious disadvantage of this method. After some preliminary 

 studies, the orderly experiments were commenced about the middle of 

 February last, and have since been carried on from day to day without 

 intermission up to the end of May. 



A number of thin wires and of thick bars of iron and steel were 

 experimented upon. The accompanying sketch (Plate 9) shows the 

 arrangement of the apparatus employed in experimenting on thin wires. 

 A reflecting magnetometer, M, which consists of a mirror carrying at 

 its back three small magnets and suspended by a single silk fibre 

 about 5 centims. long, was placed on a convenient stand nearly 

 2 metres above the floor of the laboratory. S is a white paper screen 

 divided into half millimetres, and bent into a circular arc of a metre 

 radius. It is fixed at a distance of exactly 1 metre from the 

 magnetometer, and was used to observe the deflections of the 

 magnetometer needle, which were read by the image of a fine wire 

 fixed vertically in front of a paraffin lamp, L, secured just behind the 

 scale as in a Thomson reflecting galvanometer. N is a magnet of 

 semicircular shape meant to control the strength of the field at the 

 point where the magnetometer needle is suspended. It was mounted 

 on a suitable stem in front of the magnetometer needle, with its 

 length in the plane of the magnetic meridian, and at a certain distance 



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