398 



Professors Ayrton and Perry on 



Distance in 



inches between 



the near end of 



the bar and the 



centre of the 



test-needle. 



Ql 



4 . 



5 . 



6 . 



7 . 



8 . 



9 . 

 10 . 



No. 1. 



No. 2. 





3S T c 



.3. 







Nc 



.4. 





Covered 



Bare 



Covered 



Bare 







end. 



end. 



end. 



end. 



Def. 



Tan. 



Def. 



Tan. 



Def. 



Tan. 



Def. 



Tan. 



Def. Tan. 



Def. 



Tan. 



79° 



5-14 



77° 



4-33 



82° 



7-12 



57° 



1-54 



67° 



2-30 



27° 



0-57 



77 



4-33 



71 



2-9 



77 



4-33 



53 



1-33 



62 



1-88 



21 



0-38 



69* 



2-67 



58 



1-6 



66 



2-24 



46 



1-04 



52 



1-28 



14 



0-28 



59 



1-66 



47 



1-07 



56 



1-48 



39 



0-81 



43 



0-93 



11 



0-19 



50 



1-19 



37 



0-76 



46 



1-04 



32 



0-62 



36 



0-73 



9 



016 



42 



09 



30 



0-58 



37* 



0-79 



27^ 



0-52 



29 



0-56 



7 



012 



35 



0-7 



24 



046 



30 



0'58 



18 



0-32 



22 



0-48 



4 



0-07 



30 



0-58 20 



0-36 25 



0-47 



13 



0-23 



17 



031 



3 



0-05 



To ascertain the distribution of the lines of force, iron 

 filings were sprinkled on paraffined paper, and the positions 

 the filings took up fixed by the paraffin being softened by a 

 heated piece of copper being passed over the paper at a short 

 distance above it. These fields of force are shown in the 

 diagrams 6, 7, 8, and 9(P1. IX.). From the curves in fig. 5 and 

 from the iron-filing curves it is seen that the effect of coning 

 the wire is to produce a strong field very near the pole, but 

 that the force falls off very rapidly as the distance from the 

 pole increases. With No. 2 magnet, for instance, the field 

 between the poles is so weak that scarcely any definite arrange- 

 ment of filings is traceable in the diagram 7 corresponding 

 with it. 



From the curves in fig. 5 it is seen that, at considerable 

 distances from the end of the electromagnet, 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, corre- 

 sponding with the curve C C, produces the strongest field, 

 the next strongest being produced by the magnet No. 2 with 

 the wire coned towards each end, since obviously the curve 

 BBB cuts the curve AAA at a point corresponding with 

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

 distances very close to the magnet, this method of experiment- 

 ing cannot, of course, be employed to measure the resultant 

 force accurately; and hence observations by this method at 

 distances of less than 3J inches from the end of the magnet 

 to the centre of the oscillating needle were not made, and 

 conclusions as to the resultant magnetic force very close to 

 the poles must, of course, not be drawn from the curves in 

 fig. 4. 



Keturning to the curves taken up by the iron filings, we 

 see that No. 1 magnet gives an arrangement similar to that 



