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LIX. On winding Electromagnets. By Professors W. E. 

 Aykton, F.R.S., and John Pekky, M.E* 



[Plates VIII. & IX.] 



THE following experiments were made to determine which 

 mode of winding a given length of wire on an iron bar 

 gave the strongest electromagnet for the same current. Four 

 bars of iron, each 12 inches long, were cut from the same 

 rod | inch thick; and an exactly equal length of wire was 

 wound on the four bars respectively, in the following way : — 



1. Wire wound equally over the whole length (PL VIII. 

 fig. 1). 



2. Wire coned towards each end (fig. 2). 



3. Wire wound equally over half the iron bar, leaving the 

 other end bare (fig. 3). 



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

 (fig. 4). 



Electromagnet 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 electromagnet No. 1, when magne- 

 tized 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 on No. 1, and the deflection of the 

 little needle observed when No. 1 was placed at different dis- 

 tances from the centre of the test-needle, the axis of No. 1, 

 however, always remaining in the same line. Under these 

 circumstances it is well known that the strength of the field 

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

 mately proportional to the tangent of its deflection. Experi- 

 ments were now made in a similar way with electromagnet 

 No. 2, and with each end of No. 3 and of No. 4, the same 

 current as was employed with electromagnet No. 1 being used 

 in all cases, and which was much below the saturating current. 



The results obtained are given in the accompanying table, 

 and are shown plotted in the accompanying curves (fig. 5), 

 vertical distances representing the distance between the near 

 end of the electromagnet and the centre of the test-needle, 

 and horizontal distances the tangents of the deflection of the 

 test-needle : A A A A is that for No. 1; B B B B for No. 2; 

 CCCC for the covered end of No. 3 ; DDDD 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. 



* Communicated by the Physical Society of London ; read December 9, 



1882. 



