258 Mr. E. Taylor Jones on 



being to measure the induction from the values of the 

 tractive force, using Maxwell's expression. 



Quite recently an important paper has been published by 

 Threlfall*. giving an account of experiments made with appa- 

 ratus essentially similar to Bosanquet's ; but the interfaces of 

 the electromagnets were carefully ground and polished, and 

 the tractive force was measured by a spring-balance. 



The results for high inductions do not show better agree- 

 ment than Bosanquet's with Maxwell's theory ; but the 

 author explains an important source of error which exists 

 especially in working at low inductions, viz. — that the inter- 

 faces of the magnets do not generally remain in contact until 

 the stress is completely overcome, but separate at one side 

 first, thus enclosing a " wedge-shaped gap." The numbers 

 given, however, only extend over a range of inductions from 

 11,000 to 16,000 C.G.S. units. Most of the paper deals with 

 the case when the bars are separated by layers of non- 

 magnetic substance. 



It was clear, therefore, that more accurate experiments were 

 necessary to determine whether Maxwell's expression repre- 

 sents exactly the tractive force, and if so, how apparatus is to 

 be arranged so as to allow of the accurate measurement of 

 induction by tractive experiments. 



Present Experiments. Apparatus. 



The following experiments were begun in October 1893. 

 The apparatus was devised with a view to realizing as closely 

 as possible the conditions under which the results could best 

 be compared with Maxwell's theory, and was prepared and 

 arranged as follows : — 



A chosen bar of soft German iron was turned accurately to 

 an ellipsoid of revolution of length 22' 5 7 centim. and least 

 diameter 1*5 centim. Its shape was tested by measuring its 

 volume by weighing in air and water, and by calculating the 

 volume from the above values of the axes. The two values 

 agreed to within a tenth per cent. The ellipsoid was after- 

 wards cut through in the equatorial plane, the diminution of 

 length being measured by observing the distance between two 

 marks on its surface. 



Two exactly similar magnetizing-coils were made, each 

 20 centim. long., and having 12 layers of 70 turns each 

 of 2 millim. aluminium wiref- 



* PHI. Mag. July 1894. 



t Aluminium wire was used that the coils might be as light as pos- 

 sible, it being thought desirable not to use the compensating arrangement 

 adopted by Bosanquet, as this probably introduced errors due to friction. 



