82 



NATURE 



[^Nov. 28, 1878 



almost parallel paths. If our supposed particle of north- 

 seeking magnetism were placed on a line near one pole 

 it would not pass over from pole to pole, but would 

 follow the line where it swerves away to the side. We 

 know (by experiment) that two north-seeking poles repel 

 one another, and we see here that the lines of force of 

 two such poles never run into one another, but turn aside 

 mutually repellant. You will find that this is always the 

 case when two poles repel. 



Such experiments as these led Faraday to enunciate 

 several simple yet most important principles concerning 

 the lines of force, by means of which we can learn from 

 the lines what kind of action they will produce, whether 

 attraction, or repulsion, or rotation. Firstly. — All lines 

 ■of force tend to shorten themselves. If the lines running 

 across in our first figure were replaced by actual threads 

 of stretched elastic material, we see that any " shorten- 

 ing" of them would bring the poles nearer together, 

 which, indeed, is precisely the tendency of the magnetic 

 attraction between the poles. Secondly. — Lines of force 

 repel each other when placed side by side. If this be 

 the case then the lines in our second figure, which 

 bend outwards, and run off side by side, repel one 

 another, the two poles must be experiencing a tendency 

 to move away from one another ; and this we know is 

 the case. Thirdly, Like magnetic lines of force, when 

 ■"end on" to each other, run into each other; while, 

 unlike magnetic lines, when end on, repel each other. 

 Here, of course, we apply the terms "like" and "un- 

 like " to the cases of the directions in which our supposed 

 particle of north-seeking magnetism would move along 

 the lines. These notions of Faraday's are full of 

 ■meaning, and it is not many years since Prof. Clerk Max- 

 well showed how well they agreed with the most perfect 

 mathematical expression of the forces that operate in the 

 medium filling the surrounding space. 



Keeping these simple principles in mind, let us apply 

 them to some further cases of magnetic action, and see if 

 they are equally applicable. We know that the wires 

 carrying electric currents possess certain magnetic pro- 

 perties, and will deflect magnetic needles ; that two 

 electric currents may attract or repel each other ; and that 

 current may make a magnet pole rotate round it. Can 

 we explain such electrodynamic actions also by applying 

 the principles of Faraday to the magnetic lines of force 

 existing in these rarious cases ? 



In the first place, what are the lines of force belonging 

 to a wire through which an electric current is passing ? 

 To ascertain this we will bore a hole through a card or a 

 piece of glass, and pass a wire up through the hole. 

 Then, joining the ends of the wire to the poles of a 

 powerful battery, we will, while the current is passing, 

 sprinkle on iron filings, and, tapping lightly, will permit 

 them to assume their places in the lines of force. Fig. 3 

 was thus obtained. It shows us a series of concentric 

 •circles. If a supposed north-seeking magnetic particle 

 were put down on one of these circles it would more 

 round and round in one direction ; supposing the current 

 to come up through the hole, this direction would be 

 opposite to that of the hands of a watch. If the current 

 went down through the hole, the movement would be the 

 other way round. But we may examine the current in 

 another way. Lay the conducting-wire down flat, and 

 place over it the card or piece of glass. The forms 

 assumed by the iron filings are in this case (Fig. 4) 

 straight lines across the wire — are edge-views, so to 

 -speak, of the systems of circles we just now ob- 

 tained. 



These two figures were discovered by Faraday, and 

 are given in his researches. They are also given by Dr. 

 F. Guthrie m his book on " Magnetism and Elec- 

 tricity." 



If we wind up our conducting-wire into a simple knot 

 «r loop, carefully preventing the overlapping parts from 



touching, the figure obtained with the iron filings is like 

 that of Fig. 5. It is interesting to observe how in the 

 middle of the loop there are no lines, only dots. The 

 lines of force run through the loop, perpendicularly to its 

 plane, and we only see them end-ways as points. It is 

 clear that a magnetic particle such as we have imagined 

 would be either attracted into the middle of the loop, or 

 would be repelled out of it, according to its polarity. 



Now what is the effect of carrying two parallel currents 

 through two wires side by side ? Take a piece of card or 

 glass, as in Fig. 6, having two holes in it ; through these 

 pass a couple of wires joined to two batteries, so that the 

 two currents are either both ascending or both descend- 

 ing through the flat surface. The magnetic field mapped 

 out by the iron filings will then show a series of curves, 

 the outermost of which are rough ovals inclosing both the 

 currents, whilst the innermost are small ovals round 

 each wire. The lines between the inner and outer 

 systems present a sort of hour-glass shape or lemniscate. 

 Had the two parallel currents, however, passed in oppo- 

 site directions through the plate, one ascending and the 

 other descending, the filings in the magnetic field would 

 have taken the form given in Fig. 7. Here we find two 

 separate systems of distorted and flattened circles sur- 

 rounding the wires, each separate system of circles 

 having displaced the other. The outer curves do not 

 run into each other as in the preceding case. Let us 

 apply Faraday's principles to these two figures. In the 

 former (Fig. 6) any "shortening" of the exterior lines 

 would tend to draw the centres nearer together. In the 

 latter case (Fig. 7) no such consequence need result. A 

 tendency of the successive lines to repel each other and 

 to maintain equal distances from each other, would in 

 the former case tend to reduce the entire figure to a 

 system of concentric circles, which could not be accom- 

 plished unless the two centres approached each other 

 and coalesced. In the latter case, since the systems of 

 lines round the two centres never join across, this ten- 

 dency would have the result of driving the two centres 

 far apart to allow of the lines becoming perfect sets of 

 circles. Now we know from Ampere's classical re- 

 searches on parallel currents, that they attract one 

 another when they run in the same direction, but are 

 mutually repellant when they run in opposite directions. 

 Our application of Faraday's principle enables us to 

 foresee this electro-dynamical action as a consequence of 

 the distribution of magnetic force in the field. In an 

 exactly similar manner we may reason out the action of 

 the forces in the field which is produced by two currents 

 crossing one another at a right-angle (Fig. 8), the con- 

 ducting wires attracting one another across those quad- 

 rants in which the currents flow both towards or both 

 from the point of intersection. 



We may apply our study further and investigate, with 

 iron-filings, the action which currents exert on magnets. 

 Let us conduct a current vertically through a hole in a plate, 

 and fix near it a small magnetic needle, as in Fig. 9. 

 The needle has been placed so as to point with one pole 

 towards the current. The lines of force radiating from 

 that pole run round and coalesce on one side with the 

 circular lines of force of the current. On the other side 

 of the pole they absolutely refuse to unite with the circles, 

 and repel them away. Clearly, the " tendency to shorten," 

 which Faraday predicated of the lines, would drag the 

 pole of the magnet in one direction round the current. 

 Looking at the other pole of the magnet we see that the 

 tendency acts in the opposite direction, so that the total 

 result would be a tendency to turn round the magnet 

 about its middle point, and set it at right angles to its 

 present position. This consequence, too, is, as we know 

 from Oersted's famous experiments, the fact. 



If, instead of laying the needle down flat, we had 

 reared it up on end, as in our Fig. 10, where a square black 



