loS 



NATURE 



[Dec. I, 1887 



Rotation of a Magnet by a Current. 

 The easiest way to show the actual rotation of a 

 magnet is to send a current half way along it and back 



Fig. 2=;— Round bright steel bar-magnet pivoted at its ends, spinning 

 rapidly on its axis under the influence of a current supplied to either the 

 bottom or top pivot, or both, and removed near the middle by a scrap of 

 tinfoil lightly touching it. 



outside. Thus, take a small, round, polished steel bar- 

 magnet with pointed ends, pivot it vertically, and touch 

 it steadily with two flakes or light pads of tin-foil, one 



Fig. 26. — Another mode of exhibiting the same thing as Fig. 25. The magnet 

 is loaded so as to float upright in mercury. 



near either end and one near the middle ; supply a 

 current by these contact pieces, and the magnet spins 

 with great rapidity. Reverse the current, and it rotates 



Fig. 27. — The converse of Fig. 25. Spinning the magnet mechanically give 

 a current between two springs, cne touching it near or beyond eithe 

 end, the other touching it near middle. 



the other way. Conversely, by producing the rotation 

 mechanically a current will be excited in a wire joining 

 the two pieces of tin-foil (Figs. 25, 26, and 27). 



Many more variations of the experiment could be 

 shown, but these are typical ones, and will suffice. They 

 all call attention to the fact that, a magnet, considered 

 electrically, is a rotatory phenomenon. 



Ampere's Theory. 



The idea that magnetism was nothing more nor less 

 than a whirl of electricity is no new one — it is as old as 

 Ampere. Perceiving that a magnet could be imitated by 

 an electric whirl, he made the hypothesis that an electric 

 whirl existed in every magnet and was the cause of its 

 properties. Not of course that a steel magnet contains 

 an electric current circulating round and round it, as an 

 electro-magnet has : nothing is more certain than the 

 fact that a magnet is not magnetized as a whole, but that 

 each particle of it is magnetized, and that the actual 

 magnet is merely an assemblage of polarized particles. 

 The old and familiar experiment of breaking a magnet 

 into pieces proves this. Each particle or molecule of the 

 bar must have its circulating electric current, and then 

 the properties of the whole are explained. 



There is only one little difficulty which suggests itself 

 in Ampere's theory — How are these molecular currents 

 maintained .'' Long ago a similar difficulty was felt in 

 astronomy — What maintains the motions of the planets ? 

 Spirits, vortices, and other contrivances were invented to 

 keep them going. 



But in the light of Galileo's mechanics the difficulty 

 vanishes. Things continue in motion of themselves 

 until they are stopped. Postulate no resistance, and 

 motion is essentially perpetual. 



What stops an ordinary current 1 Resistance. Start 

 a current in a curtain ring, by any means, and leave it 

 alone. It will run its energy down into heat in the space 

 of half a second or so. But if the metal conducted in- 

 finitely well there would be no such dissipation of energy, 

 and the current would be permanent. 



In a metal rod, electricity has to pass from atom to 

 atom, and it meets with resistance in so doing ; but who 

 is to say that the atoms themselves do not conduct per- 

 fectly ? They are known to have various infinite proper- 

 ties already : they are infinitely elastic, for instance. 

 Pack up a box of gas in cotton-wool for a century, and 

 see whether it has got any cooler. The experiment, if 

 practicable, should be tried ; but our present experience 

 warrants us in assuming no loss of motion among the 

 colliding atoms until the contrary has been definitely 

 proved by experiment. To all intents and purposes 

 certainly atoms are infinitely elastic : why should they 

 not also be infinitely conducting .-^ Why should dissipa- 

 tion of energy occur in respect of an electric current 

 circulating wholly inside an atom ? There is no known 

 reason why it should. There are many analogies against 

 it. 



How did these currents originate .'' We may as well 

 ask. How did any of their, properties originate "i How 

 did their motion originate ? These questions are un- 

 answerable. Suffice it for us, there they are. The atoms 

 of a particular substance — iron for instance, or zinc — 

 have an electric whirl of certain strength circulating in 

 them as one of their specific physical properties. 



This much is certain, that the Amperian currents are 

 not producible by magnetic experiments. When a piece 

 of steel or iron is magnetized, the act of magnetization is 

 not an excitation of Amperian current in each molecule 

 — is not in any sense a magnetization of each molecule. 

 The molecules were all fully magnetized to begin with : 

 the act of magnetization consists merely in facing them 

 round so as to look mainly one way — in polarizing them, 

 in fact. This was proved by Beetz long ago ; I will not 

 stop to explain it further, but v/ill refer students to 

 Maxwell. 



