346 



NATURE 



{Feb. 9, 1888 



Whether the medium be conducting or insulating makes 

 no difference to the general fact of spinning wheels inside 

 it wherever lines of force penetrate it ; but the wheels of a 

 conductor are imperfectly cogged together, and accordingly 

 in the variable stages of a magnetic field, while its spin is 

 either increasing or decreasing, there is a very important 

 distinction to be drawn between insulating and conducting 

 matter. During the accelerating era conducting matter 

 is full of slip, and a certain time elapses before a steady 

 state is reached. A certain time may be necessary for 

 the propagation of spin in a dielectric, but it is excessively 

 short, and the process is unaccompanied by slip, only by 

 slight distortion and recovery. As for a strongly magnetic 

 substance like iron, nickel, or cobalt, one must regard 

 them as constituted in the same sort of way, but with 

 wheels greatly more massive, or very much more 

 numerous, or both. 



Phenomena connected with a varyhtg Current. Nature 

 of Self-induction. 



Proceed now to think what happens in the region 

 round a conductor in which a current is rising. Without 

 attempting a complete and satisfactory representation of 

 what is going on, we can think of some mechanical 

 arrangements which have some analogy with electrical 

 processes, but do not pretend to imitate them exactly. 



Take first a system of wheel-work connected together 

 and moved at some point by a rack. Attend to alternate 



Fig. 39.— a p-ovisional representation of a current surrounded by dielectric 

 medium, either propelling or being propelled. 



wheels more especially, as representing positive elec- 

 tricity. The intermediate negative wheels are necessary 

 for the transmission of the motion, and they also serve to 

 neutrahze all systematic advance of positive electricity in 

 any one direction, except where slip occurs, but they 

 need not otherwise be specially attended to. 



Remember that every wheel is endowed with inertia, 

 like a fly-wheel. 



Directly the rack begins to move, the wheels begin to 

 rotate, and in a short time they will all be going full 

 speed. Until they are so moving, the motion of the 

 rack is opposed, not by friction cr ordinary resistance, 

 but by the inertia of the wheel-work. 



This inertia represents what is called self-induction, 

 and the result of it is what has been called the " extra 

 current at make," or, more satisfactorily, the opposing 

 E.M.F. of electro-magnetic inertia or self-induction. 



So long as the rack moves steadily forward, the wheel- 

 work has no further effect upon it ; but directly it tries to 

 stop, it finds itself unable to stop dead without great 

 violence : its motion is prolonged for a short time by the 

 inertia of the wheel-work, and we have what is known as 

 the " extra current at break." 



If the rack is for a moment taken to represent 

 the advancing electricity in a copper wire, then the 

 diagram may be regarded as a section of the complete 



field : the complete field being obtained from it by rotat- 

 ing it round the axis of the wire. Imagining this done, 

 we see that the axis of each wheel becomes prolonged 

 into a circular core, and each wheel into a circular vortex 

 ring surrounding the rack and rolling down it as it moves 

 forward, as when a stick is pushed through a tight-fitting 

 umbrella-ring held stationary (see Fig. 30 b). 



As one goes further and further from the rack the 

 lengths of the vortex cores increase, but there is only a 

 given amount of rotation to be shared among more and 

 more stuff, hence it is not difficult to imagine the rate of 

 spin diminishing as the distance increases, so that at a 

 reasonable distance from the conductor the medium is 

 scarcely disturbed. 



To perceive how much rotation of the medium is 

 associated with a given circuit, one must consider the 

 shape of its contour — the position of the return current. 

 Take first a long narrow loop and send a current up one 

 side and down the other. The rotations belonging to 

 each are superposed, and though they agree in direction 

 for the space inclosed by the loop, they oppose each other 

 outside, and so there is barely any disturbance of the 

 medium outside such a looped conductor ; very little 

 dielectric is disturbed at all, and accordingly the inertia 

 or self-induction is very small. 



If the loop opens out so as to inclose an area, as the 

 centrifugal force of the wheels will tend to make it do, 



Fio. 40. -^Diagram of a direct and return current close together, showing 

 distribution of rotation and of slip in tie thickness of t'le conductor, 

 and in the dielectric between. The dielectr.c outside is very little 

 disturbed. 



then there is a greater amount of rotation, a greater 

 moment of momentum inside it, and accordingly its 

 self-induction is increased. The axis of every wheel is, 

 however, continuous, and must return outside the loop : 

 so the outside region is somewhat affected by rotation, 

 but of a kind opposite to that inside. 



Figs. 38 and 41 show the state of things for a closed 

 circuit conveying a current. The free space in Fig. 38 

 represents a perfect conductor, or perfect breach of con- 

 nection. Along one side of this space positive electricity 

 is seen streaming in the direction of the arrows, and it 

 may be streaming on the other side also, but nothing 

 happens in its interior — which is therefore not represented. 



The corresponding portion in Fig. 41 is intended for an 

 ordinary conductor, full of wheels capable of slip. And 

 slip in this case is a continuous necessity, for the rotation 

 on either side of the conductor is in opposite directions, 

 so the atoms of the conductor have to accommodate 

 themselves as best they can to the conditions ; some of 

 them rotating one way, some the other, and some along a 

 certain neutral line of the conductor being stationary. If 

 a conductor is straight and infinitely long, the neutral 

 line of no rotation is in the middle. If it be a loop, the 

 neutral line is nearer the outside than the inside, because 

 the rotation of the medium inside is the strongest. If the 



