492 



Mr. S. Bidwell. On the Lifting Power of [June 10, 



number of turns was, therefore, 1929, and when the two halves were 

 fitted together the coil was practically continuous. 



The mean radius of the divided ring being 3" 76 cm., its mean cir- 

 cumference is 23'6 cm. Hence the magnetic force produced by a 

 current through the coil of C amperes = 102*7 C. 



The experiment was made in the same manner as before, but the 

 length of the wire contained in the ring- coil was about twice the 

 length of that in the straight solenoid, and in consequence of the 

 increased resistance thus introduced and other differences in the con- 

 ditions of the arrangement, the greatest magnetic force which could 

 be produced by the whole battery of 27 cells did not exceed 585 units. 

 On the other hand, it was found possible to obtain satisfactory results 

 with smaller currents than could be used in the former case. When 

 the magnetic force was less than 40 units the residual or permanent 

 magnetism of the divided rod seriously interfered with the observa- 

 tions ; but in the experiment with the divided ring, owing partly 

 perhaps to the superior quality of the iron, and partly to the difference 

 of the arrangement, the magnetic force could be diminished to about 

 4 units before the residual magnetism began to be troublesome.* 



The results of the experiment are given in the third and fourth 

 columns of Table I, and plotted in fig. 1 as a curve which from 

 f = 240 is sensibly a straight line represented by W = 53 g + 12,800. 



It occurred to me that if an expression could be found which 

 would satisfactorily connect the magnetic force and the weight lifted 

 with the magnetic intensity of the iron ring, the results above 

 obtained might be applied to the investigation of the changes of 

 magnetisation which correspond to changes of magnetic force. "j" The 

 common belief that at a comparatively early stage the intensity of 

 magnetisation becomes sensibly constant is, I imagine, founded rather 

 upon inference than upon actual observation. At all events, I am 

 acquainted with no experiments bearing upon the subject which have 

 been made with magnetic forces at all comparable in magnitude with 

 those used by myself. 



If from a portion of a magnet in which the direction and intensity 

 of the magnetisation are uniform there be hollowed out a cavity in 

 the form of a thin disk whose plane is normal to the direction of 

 magnetisation, a unit magnetic pole at the middle of the axis 

 experiences a force 47rl due to the attraction of the superficial 

 magnetism of the disk on the positive side, and the equal repulsion 



* If it did in fact exert any material influence before this point was reached, the 

 effect was at all events uniform, and not apparently capricious. 



f The expression used by Rowland in the paper before referred to is 

 W = Q 2 / 246 5 5 00000 kilograms per square centimetre, which translated into C.Gr.S. 

 units and the ordinary notation, becomes W = ( § 2 /87r^ grams per square centimetre. 

 This is clearly erroneous, 



