154 Dr. E. Taylor Jones on 



Apparatus. 



These inductions were obtained by means of the Ring 

 Electromagnet designed by Dr. H. du Bois *. 



The chief parts of the apparatus are shown in fig. 1, by a 

 vertical axial section ; the left half is not all shown, but is 

 symmetrical with the right. The pole-pieces were conical, 

 and had a vertical angle of 78° 28', so that the field near the 

 vertex was as nearly uniform as possiblef. 



Both pole-pieces were axially perforated, the width of the 

 openings being about 5 millim. 



Into one of the pole-pieces fitted a short cylindrical iron 

 bar, Ci, provided with a flange, so that when pressed home 

 it just reached to the vertex of the cone. Into the opening 

 in the other pole-piece fitted a longer bar, C 2 , of the same iron 

 as Ci and of equal diameter ; this bar could be moved to and 

 fro in its socket with very little friction. The two bars thus 

 met in the equatorial plane EE' and formed an " isthmus " 

 between the pole-pieces. Similar bars were also made of 

 steel. 



This part of the apparatus was carefully centred, and the 

 inner ends of the bars were turned plane and polished ; in 

 this process the pole-pieces themselves served as " guard- 

 rings " before they were completely turned down to their 

 conical form. The surfaces w r ere optically tested as in the 

 former experiments {I.e. p. 262). 



The bar C 3 was connected, by a long brass rod M and a 

 cord which passed over a pulley, with a scale-pan, so that the 

 tractive force could be directly measured. For measuring 

 the induction and magnetic force in the isthmus the method 

 of Ewing and Low was employed. A small brass bobbin, on 

 which two coils, each of five turns of thin copper wire, were 

 wound, could be placed on the isthmus near the plane of 

 section, so that when the bar C 2 was pulled out the bobbin 

 automatically fell out to a distance and was turned so that 

 its plane remained parallel to the field. The diameter of one 

 of the coils was slightly greater than that of the isthmus, 

 that of the other about 2 millim. greater, so that an annular 

 space about 1 millim. wide existed between the coils. Each 

 coil could be connected through a resistance-box with an 

 Ayrton-Mather galvanometer. The movable coil of this 

 instrument was specially weighted to make it suitable for 

 ballistic measurements. 



* Du Bois.. Wied. Ann. li. p. 537 (1894) ; see fig. 11 there. 



f Ewicg, ' Magnetic Induction in Iron and other Metals,' § 97 (18C3). 



