winding Electromagnets. 399 



of an ordinary regularly magnetized bar-magnet. With No. 

 2 the lines around the poles are similar to those of No. 1, but 

 the field between the poles is very weak. Magnets Nos. 3 

 and 4 give very similar figures, showing a very peculiar 

 distribution of force. There is a great concentration of the 

 lines at the pole corresponding to the half of the iron which 

 is covered with wire ; but the unwound end seems to form a 

 long weak pole, with its maximum force near the centre of the 

 bar, i. e. at the inner end of the coil, — the differences between 

 these two being, that with No. 4 magnet there is, compara- 

 tively, a greater concentration of force at the wound pole, and 

 that the opposite pole is longer and extends a little way into 

 the coil — the result of the coning of the wire. In these two 

 cases the unwound end of the iron seems to act like an arma- 

 ture. 



To ascertain the force which each magnet would exert 

 on an armature, experiments were made and the following 

 results obtained, the current flowing through the coil in each 

 case being exactly the same, as well as the armature em- 

 ployed : — 



iy , , Weight required to detach the armature 



a ° e ' from the covered end of the magnet. 



No. 1 45 ounces. 



2 57 „ 



3 57 „ 



4 77 „ 



These results confirm those previously obtained, that the 

 field produced by the covered ends of the electromagnets 

 numbers 2 or 3 at distances near the pole is much stronger 

 than that produced by No. 1. But they show something else, 

 viz. that for very small distances it is the covered end of 

 No. 4 that produces the strongest field. In other words, 

 returning to fig. 5, the curve EE, although much below the 

 curves A A, BB, and C C, must rise rapidly and cut the others, 

 just as the curve C C cuts the curve A A, at a point corre- 

 sponding with a distance of about 4" 2 inches from the end of 

 the magnet, and just as, again, the curve B B cuts A A at a 

 point corresponding with a distance of about 3*2 inches from 

 the end of the magnet. The curves of iron-filings (fig. 9) 

 indeed give indication of the great strength and concentration 

 of field there is produced close to the iron by the wire coned 

 at the end, as employed in the magnet No. 4. 



With, then, a definite iron core, a definite length of wire to 

 be coiled on it, and to be traversed with a definite current, the 

 mode of coiling to produce the largest field depends entirely 

 on the distance from the end of the electromagnet at which 



