174 



♦ KNOWLEDGE ♦ 



[Aua. 29, 1884. 



4. Wire wound on one half, but coned towards the end 

 (Fig. 4). 



Fig. 4. 



To ascertain their relative strengths, electro-magnet 

 No. 1 was put so that its axis was at right angles to the 

 axis of a small magnetic needle and passed through the 

 point of suspension of the needle, which was suspended so 

 as to move freely in a horizontal plane, and far enough 

 away that the magnetic field due to the electro-magnet, 

 when magnetised by passing a current through it, was 

 nearly constant over that portion of the field in which the 

 little suspended needle moved when deflected. A constant 

 current was now passed through the coil, and the deflection 

 of the little needle observed when the electro-magnet was 

 placed at different distances from the centre of the test- 

 needle, the axis, however, always remaining in the same 

 line. Under these circumstances, the strength of the field 

 produced by No. 1 at the centre of the test-needle is 

 approximately proportional to the tangent of the angle 

 through which the needle is deflected. Experiments were 

 made in a similar way with electro-magnet No. 2, and with 

 each end of No. 3 and of No. 4, the same current being 

 used in all cases. 



TANGENT Or OErLECTlQN OF NBtDLE 



The results obtained are shown plotted in the accompany- 

 ing curves (Fig. 5), vertical distances representing the dis- 

 tance between the near end of the electro-magnet and the 

 centre of the te.st-needle, and horizontal distances indi- 

 cating the tangents of the deflections of the test^needle : 

 A AAA is that for No. 1 ; B B B B for No. 2 ; C CO 

 for the covered end of No. 3 ; D D D D for the uncovered 

 end of No. 3 ; E E E E for the covered end of No. 4 ; and 

 F F F F for the uncovered end of No. 4. 



These cur\'es show that at considerable distances from 

 the end of the electro-magnet the uniformly coiled magnet 

 No. 1 produces the most powerful field, while for points 

 nearer the magnet, but still at a distance of about 3 inches 

 from it, the covered end of No. 3 magnet, corresponding with 

 the curve C, produces the strongest field, the next in 

 strength being produced by the magnet No. 2, with the 

 wire coned towards each end, since obviously the curve 

 B B B cuts the curve A A A at a point corresponding with 

 a distance of about 3 inches from the end of the magnet. 

 The strength of the field at shorter distances than those 

 indicated cannot be measured by the means above adopted. 

 There is, however, another, although perhaps a rougher, 

 means of observing the strength of the various parts of 

 the field produced by the electro-magnet. It is well known 



that a magnet is capable of attracting iron filings, and that 

 they take up definite positions when attracted. It is also 

 an every-day experimeut to lay a magnet under a sheel of 

 paper, thin glass, or other non-magnetic material, upon 

 which iron filings are then sprinkled, when they form 

 certain definite figures. If the magnet is a straight bar of 

 steel, the filings take up their position in a manner ana- 

 logous to that depicted in Fig. G. Professors Ayrton and 

 Perry pursued a similar course in investigating the mag- 

 netic properties of their coils, and obtained some interesting 

 and instructive results. 



Fij?. 6. 



^-^--■r..:M 



Fig. 7. 





Fig. 8. 





Fig. 9. 



Figs. 6, 7, 8, 9 afford remarkable illustrations of the 

 effects produced. The iron filings are seen to take up defi- 

 nite positions, and supposing Fig. 6 to represent an ordinary 

 bar magnet, there is observable a concentration of filings 

 around the extremities indicating the positions of maximum 

 magnetic strengtk It will also be noticed that £J1 the 

 filings set themselves in curves, and that these cur\'es 

 exhibit a strong tendency to connect the one pole with the 

 other, much in the same way that they do when the poles 

 of an ordinary horse-shoe magnet are dipped into a quan- 

 tity of filings. Even the apparently straight lines taken 

 by the filings at the extremities (Fig. 6) are in reality only 

 parts of large curves, easily discernible in the actual 

 experiment. 



