Feb. 9, 1888] 



NATURE 



347 



loop be shut up to nothing, the neutral line is its outer 

 boundary or nearly so (Fig. 40). If, again, the circuit is 

 wound round and round a ring, as string might be lapped 

 upon a common curtain-ring to cover it, then the axes of 

 whirl are wholly inclosed by the wire, and there is no 

 rotation outside at all. 



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Fig. 41.— Diagram of simple conducting circuit like a galvanometer ring, 

 with the alternate connecting- wheels omitted. The same number of di- 

 electric wheels are drawn outside as inside, to indicate the fact that the 

 total spin is equal inside ar.d out, though the outside is so spread out as 

 to be much less intense. 



Fig. 42 shows a section of this last-mentioned condition, 

 and here the wheels of the dielectric outside are not 

 rotating at all. The inside is revolving, it may be furiously, 

 and so between the inner and outer layers of the conductor 

 we have a great amount of slip and dissipation of energy. 



Fig. 42. — Section of a closed magnetic circuit, or electric vortex-ring, or 

 hollow bent solenoid hke Fig. 29, inclosing an anchor-ring airspace; 

 the axis of the ring being A B, the sections of the core being H and F. 

 The arrows indicate the intensity of the spin, i.e. of the magnetic field, 

 which is a maximum at the middle of each section and nothing at all 

 outside. If the core contains iron instead of air, its wheels have to be 

 from ICO to 300 times as massive : slipping wlieeli if solid iron, cogged 

 wheels if a bundle of fine varnished iron wires. 



The process of slip which we have depicted goes on in 

 all conductors conveying a current, whether steady or 

 variable, and in fact is the current. The slip is neces- 

 sarily accompanied by dissipation of energy and pro- 

 duction of heat : only in a perfect conductor can it occur 

 without friction. In a steady current the slip is uniformly 

 distributed throughout the section of the conductor ; in 



the variable stages it is unequally distributed, being then 

 more concentrated near the periphery of the wire. 



When a current is started in a wire, the outer layers 

 start first, and it gradually though very quickly penetrates 

 to the axis. Hence the lag or self-induction of a wire 

 upon itself is greater as the wire is thicker, and also as it 

 is made of better conducting substance. If it is of iron, 

 the mass or number of the wheels is so great that the lag 

 is much increased, and the spin of its outer layers is 

 great enough to produce the experimental effects dis- 

 covered by Prof. Hughes. 



One must never confuse the slip with the spin. Slip is 

 current, spin is magnetism. There is no spin at the axis 

 of a straight infinite wire conveying a current, and it 

 increases in opposite directions as you recede from the 

 axis either way ; arranging itself in circular vortex cores 

 round the axis. But the slip is uniformly distributed all 

 through the wire as soon as the current has reached the 

 steady state. The slip is wholly in the direction of the 

 wire. The axes of spin are all at right angles to that 

 direction. 



Rise of Induced Current iti a Secondary Circuit. 



To study the way in which a magnetic field excited in 

 any manner spreads itself into and through a conducting 

 medium, look at Fig. 43, and suppose the region inside 

 the contour A B c D to be an ordinary conducting region — 

 that is, full of wheels imperfectly geared together, and 

 capable of slip. 



Fig. 43.— Diagram illustrating the w.yy in which an induced current arise 

 in a mass of metal immersed in an increasing magnetic field ; also how 

 it decays. The dotted lines a b CD, e fg h, i j k i., are successive lines 

 of slip. 



■Directly the rack begins to move, all the wheels outside 

 A B c D begin to rotate, and quickly get up full speed. 

 The outer layer of wheels inside the contour likewise 

 begins to rotate, but not at once ; there is a slight delay in 

 getting them into full motion. For the next inner layer 

 the delay is rather greater, and so on. But ultimately the 

 motion penetrates everywhere equally, and everything is in 

 a steady state. 



But while the process of starting the wheels was going 

 on, a slip took place round the contour A B C p, and round 

 every concentric contour inside it ; the periphery of the 

 positive wheels moving in a direction opposite to that of 

 the wheel in contact with the rack, and so suggesting the 

 opposite induced current excited at "make" in the sub- 

 stance of a conductor near a growing current, or generally 

 in an increasing magnetic field. 



The penetration of the motion deeper and deeper, and 

 the gradual dying away of all slip, illustrate also the mode 

 in which this induced current arises and gradually dies 

 away, becoming nil as soon as the magnetic field {t.e. the 

 rotation) has penetrated to the interior of all conductors 

 and become permanently estabhshed there as elsewhere. 



Suppose the motion of the rack now stopped : all the 

 cogged wheels stop too, though it may be with a jerk and 



