Dec. I, 1887] 



NATURE 



109 



Ampere's Theory extended by Weber to explain 

 Diamagnetistn also. 



Let us see how far we have got. We have made the 

 following assertions : — 



(i) That a magnet consists of an assemblage of polar- 

 ized molecules. 



(2) That these molecules are each of them permanent 

 magnets, whether the substance be in its ordinary or in 

 its magnetized condition, and that the act of magnetiza- 

 tion consists in turning them round so as to face more or 

 less one way. 



(3) That when all the molecules are faced in the 

 same direction the substance is magnetically completely 

 saturated. 



(4) That if each molecule of a definite substance con- 

 tains an electric current of definite strength circulating 

 in a channel of infinite conductivity the magnetic beha- 

 viour of the substance is completely explained. 



But now, supposing all this granted, how comes it that 

 the molecular currents are not capable of being generated 

 by magnetic induction .? And if we cannot excite them, 

 are we able to vary their strength } 



The answer to these questions is included in the following 

 propositions, which I will now for convenience state, and 

 then proceed to explain and justify. 



(5) If a substance possessing these molecular currents 

 be immersed in a magnetic field, all those molecules which 

 are able to turn and look along the lines of force in the 

 right direction will have their currents weakened ; but on 

 withdrawal from the field they will regain their normal 

 strength. 



(6) If the currents flowing in the conducting channels 

 be feeble or «//, the act of immersion of the substance in 

 a magnetic field will reverse them or excite opposite cur- 

 rents, which will last so long as the body remains in the 

 field, but will be destroyed by its removal. 



(7) The molecular currents so magnetically induced are 

 sufficient to explain the phenomena of diamagneiism. 



Let us first just recall to mind the well-known elementary 

 facts of current induction. A conducting circuit, such as 

 a ring or a coil of wire, suddenly brought near a current- 

 conveying coil or a magnet, has a momentary current 

 induced in it in the opposite direction to the inducing 

 current— in other words, such as to cause momentary re- 

 pulsion between the two. So long as it remains steady, 

 nothing further happens ; but on withdrawing it another 

 rnomentary current is induced in it in the contrary direc- 

 tion to that first excited. The shortest way of expressing 

 the facts quite generally is to say that while from any 

 cause the magnetic field through a conductor is increas- 

 ing in strength a current is excited in it tending to drive 

 it out of the field : the disturbance is only temporary, but 

 whenever the magnetic field decreases again to its old 

 value a reverse flow of precisely the same quantity of 

 ■electricity occurs. Fig. 28 shows a mode of illustratino- 

 the facts. A copper disk is supported at the end of a 

 torsion arm and brought close to the face of an unexcited 

 bar electro-magnet. On exciting the magnet the disk is 

 driven violently away : to be sucked back again, however 

 whenever the magnetism ceases. ' 



Now, why are all these effects so momentary .? What 

 makes the induced current cease so soon after excitation ? 

 Nothing but dissipation of energy : only the friction of 

 imperfect conductivity. There is nothing to maintain the 

 current : it meets with resistance in its flow through the 

 metal, and so it soon stops. 



But in a perfect conductor like a molecule no such dis- 

 sipation would occur. Electricity in such a body will 

 obey the first law of motion, and continue to flow till 

 stopped. Destroying the magnetic field will stop an 

 mduced molecular current, but nothing else will stop it. 

 Hence it follows that the repulsion experienced is no 

 transitory efl"ect like that in Fig. 28, but is as permanent 

 as the magnetic field which excites and exhibits it. 



Thus, then, a body whose molecules are perfectly con- 

 ductmg, but without specific current circulating in them, 

 will behave diamagnetically, / e. will move away from strong 

 parts of the field towards weak ones, and thereby set its 

 length equatorially, just as bismuth is known to do. 



Whether this be the true explanation of diamagnetism 

 or not, It is at least a possible one. It is known as Weber's 

 theory. 



It does not necessarily follow that the specific molecu- 

 lar currents of a diamagnetic substance are really nil; 

 all that is needful is that they shall be weaker than those 

 induced by an ordinary magnetic field. By using an 

 extremely weak field, however, the specific currents need 

 not be quite neutralized, and in such a field the body 

 ought to behave as a very feebly magnetic substance. Such 

 an effect has been looked for (see Nature, vol. xxxv 

 p. 484). 



One loop-hole there is, however, viz, that every molecule 

 may be so jammed as to be unable to turn round, and 

 such a substance could hardly exhibit any noticeable mag- 

 netic properties. The molecules would have got them- 

 selves into a state of minimum potential energy, and if 

 jammed therein nothing could be got out of them. The 

 induced currents of diamagnetism would be superposed 



Fig. 28. — Stout disk of copper supported on a horizontal arm near one pole 

 of a bar electro-magnet. The disk is repelled every time the magnet 

 is excited, and is attracted while the magnetism is destroyed. 



upon them just as if no initial molecular currents existed. 

 By varying the temperature of such a substance, however, 

 one might expect to alter their arrangement, and so 

 develop magnetic properties in it, just as electrical pro- 

 perties are developed in crystals like tourmaline by heat 

 or by cold. 



We are now able clearly to appreciate this much — that 

 the molecular currents needful to explain magnetism are 

 not conceivably excited by the act of magnetization, for 

 they are in the wrong direction. Induced molecular 

 currents will be such as to cause repulsion : those which 

 cause attraction must have existed there before, and be 

 merely rotated into fresh positions by the magnetizing 

 force. 



Function of the Iron in a Magnet. Two Modes of 

 expressing it. 



We can now ex'plain the function of iron, or other 

 magnetic substance, in strengthening a magnetic field. 

 Take a circular coil of wire. Fig. 29, and send a current 

 round it : there is a certain field — a certain number of 

 lines of force — between its faces. Fill the coil with iron, 

 so as to make it a common electro-magnet, and the 

 strength of the field is greatly increased. Why t The 

 common mode of statement hkens the magnetic circuit 

 to a voltaic circuit ; there is a certain magneto-motive 



