•218 



Mr. E. Wilson. The Growth of Magnetism in 



If an ectodermal origin of the antennary and maxillary glands be 

 'Confirmed in crnstacea generally, then we should be led to regard these 

 structures as nephridia, which have lost their primitive connection with 

 a coelom, and the end-sac would be looked upon as equivalent to the 

 Li flame-cell" of a typical intracellular nephridium. 



The above preliminary account, which has omitted all reference to 

 the nervous system and sense-organs, is merely a summary of the 

 results already obtained. I hope in a future publication to give a full 

 account, containing careful drawings with the camera lucida. 



■" The Growth of Magnetism in Iron under Alternating Magnetic 

 Force." By Ernest Wilson. Communicated by Professor 

 J. M. Thomson, F.E.S. Received February 25, — Eead March 

 28, 1901. 



The object of this paper is to investigate the growth of magnetism 

 in an iron cylinder when the magnetising force is alternating. The 

 shielding effect of induced currents in plates of iron has been dealt 

 with theoretically by Professor J. J. Thomson,* and Professor J. A. 

 Ewing.f The subject has also been dealt with experimentally in the 

 case of an iron cylinder, 4 inches diameter, % with alternating mag- 

 netising force and with simple reversal of the magnetising force. A 

 cylinder, 12 inches diameter, has been experimented upon with simple 

 reversal of magnetising force,§ and the shielding effect of induced 

 currents studied. As the exploring coils enclosing elements of the 

 •cross-section of this 12-inch magnet are well suited to give the average 

 induction density at four mean radii, the author thought the subject 

 worth further investigation with regard to alternate currents. The 

 magnet is of cast steel, and is shown in sectional elevation in fig. 1. A 

 section of the 12-inch core on the line A A is given in fig. 2. Wires 

 have been threaded through the holes drilled in the plane AA, 

 enclosing the areas numbered 1, 2, 3, 4 (fig. 2), and another coil 

 (No. 5) surrounds the core. A DArsonval galvanometer was placed 

 in each of these five circuits with an adjustable resistance to control 

 the maximum deflection. The deflections of the needles of the five 

 galvanometers were noted simultaneously every four seconds, and 

 were ultimately plotted in terms of time. The magnetising current 

 in the copper coil of the magnet was observed simultaneously with the 

 above on a Weston ampere meter. The current was made to alternate 



* ' The Electrician,' vol. 28, p. 599. 

 t ' The Electrician, vol. 28 p. 631. « 



% Hopkinson and Wilson, ' Phil. Trans.' A, vol. 186 (1895), pp. 93-121. 

 § Hopkinson and Wilson, ' Journal of the Inst. Elec. Eng.,' vol. 24, p. 195. 



