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LX1V. On the External Field of Helically Magnetized Rings. 

 By W. M. Mordey*. 



ON the subject of some experiments by the present writer 

 relating to drag on armature conductors, enclosed or 

 nearly enclosed in iron, Prof. H. du Bois contributed an 

 article in the Elektrotechnische Zeitschrift of August 19 last 

 (a translation of which appeared in the ' Electrician ' of 

 August 27), in the course of which attention was particularly 

 directed to a phenomenon of ring magnets. Prof, du Bois 

 writes : — A ring-magnet experiences a side thrust when in an 

 external field whose lines of force are in the same plane as the 

 ring ; and conversely it exerts a thrust in the opposite direction 

 upon the supporter of the external field.''' u This deduction," 

 he adds, " so astonishing at first sight, I have proved to be 

 verified by experiment." As its author gives the emphasis 

 of italics to this passage, it may be well to submit what 

 appears to be the explanation, as the present writer chanced 

 to come across the effect in question a good many years ago. 

 It is, as Prof, du Bois says, rather astonishing at first sight, 

 but the explanation seems to be quite simple. The external 

 field is caused by the advancing or longitudinal constituent of 

 the helix. Magnetically a helix traversed by current acts 

 as a series of rings, but in addition it has a minor magnetizing 

 effect, at right angles to that of the rings, due to the advance 

 of the conductor from end to end of the helix. Thus, a helix 

 bent into a ring or wound about an annular core, forms an 

 endless or ring-magnet when traversed by current. But in 

 addition to this main effect, the faces of the ring, regarded 

 axially, present N. and S. polarity respectively, the whole 

 closed helix acting externally (so far as this subsidiary effect is 

 concerned), and as regards external fields, as a simple loop or 

 ring of conductor carrying current. This may be illustrated 

 by a lines-of-force figure, taken from a ring helically wound 

 with a single layer of winding. In such a figure the lines 

 of force radiate from the centre or axis of the ring at right 

 angles to the axis of the spiral — they represent portions of 

 closed lines of force which enclose the axis of the spiral. In 

 a helix wound with two layers, one forward, the other back, 

 the external magnetizing effect due to this cause is nil. In a 

 single layer (or simple) helix, this external effect may be 

 neutralized by returning the wire along the axis of the helix. 



A simple straight helix acts externally as a straight con- 

 ductor. 



A flat spiral acts like a disk or wheel, or as a number of 

 radial conductors in one plane traversed by current passing 

 from or to the centre. 



* Communicated by the Author. 



