262 Mr. E. Taylor Jones on 



The current was made, and weights added until the coil 

 returned to its original position. These added weights mea- 

 sured the total attraction of the coils. From this must be 

 subtracted the part corresponding to the area occupied by the 

 core, since this is already included in B. 



The correction was found to be negligible, never being more 

 than a sixth per cent, of the attraction of the electromagnets, 

 and for small currents less than a twentieth per cent. 



Another correction is due to the effect of the surface- 

 distribution of magnetism on the ellipsoid in so far as the 

 corresponding tubes of force pass through the air. This 

 correction, which vanishes with ring-magnets and infinitely 

 long cylinders, was found on calculation to amount to less 

 than y 1 ^ per cent. 



The interfaces of the ellipsoid were then polished by the 

 firm Hartmann and Braun of Frankfurt. In this process 

 the surfaces Mere surrounded with wide " guard-rings " to 

 keep the edges as sharp as possible. The mirrors showed 

 a so-called a black polish," and gave a perfectly clear image 

 with a 32-magnifying-power telescope and scale at a distance 

 of 5 metres. It would be hardly possible to obtain on metal 

 a nearer approach to an absolutely geometrical plane. 



Also in order better to guide the lower coil, the opening in 

 the platform P (fig. 1) was made smaller, just large enough, 

 in fact, to allow the tube to pass freely through it. A series 

 of readings was taken, as before, and these agreed consider- 

 ably better with Maxwell's theory both for low and for high, 

 but especially for low inductions. 



This I attributed chiefly to the improved guiding of the 

 lower coil ; and it was found that at low inductions, by very 

 slightly altering the position of the upper coil, a position could 

 be found in which the tractive force was a minimum, and 

 that if the applied weights were rather less than this minimum 

 value the upper coil (never being absolutely rigidly fixed), on 

 being slowly moved towards the " minimum " position, 

 suddenly jumped across it, thus showing, so to speak, a pre- 

 ference for positions in which it could support heavier weights 

 or the actual weight more easily. 



The measurements were therefore made as follows: — The 

 current being kept constant, shot was poured into the scale- 

 pan in small instalments, the upper coil after each addition 

 being carefully moved by hand until the " minimum " position 

 (easily observed by the jerk) was found. This was repeated 

 until the coil fell in this position. 



The explanation of this is given in ThrelfalPs paper (I. c), 

 in which it is shown that at small inductions the tractive force 

 is less when the two pole-faces are everywhere in contact 



