no 



NATURE 



[Dec. I, 1887 



force, and a certain resistance : the quotient gives the 

 resulting magnetic induction, or total number of lines of 

 force. Iron is more permeable than air — say, 300 times 

 more permeable — and accordingly the resistance of the 

 iron part of the circuit is almost negligible in comparison 

 with that of the air-gap between the poles. Thus a good 

 approximation to the total intensity of field is obtained 

 by dividing the magneto-motive force by the width of the 

 air-gap ; or more completely and generally by treating 

 the varying material and section of a magnetic circuit 

 just as the varying material and section of a voltaic 

 circuit is treated, and so obtaining its total resistance. 

 Iron is thus to be regarded as a magnetic conductor some 

 300 times better than air. 



This mode of regarding the case is undoubtedly simple 

 and convenient, but it is not the fundamental mode. If 

 we look at it less with a view to practical simplicity than 

 with the aim of seeing what is really going on, we shall 

 express it thus : — 



Befoie the iron Avas inserted in the coil there were a 

 certain number of circular lines of force inside it due to 

 the current alone. A piece of common iron, although 



Fig. 29. 



full of polarized molecules, has no external or serviceable 

 lines of force : they are all shut up, as it were, into little 

 closed circuits inside the iron. But directly the iron finds 

 itself in a magnetic field some of these open out, a chain 

 of polarized molecules is formed, and the lines due to its 

 molecular currents add themselves to those belonging to 

 the current of the magnetizing helix. 



Thus our ring electro-magnet has now not only its own 

 old lines of force, but a great many of those belonging to 

 the iron which have sympathetically laid themselves along- 

 side the first. 



The end result of either mode of regarding the matter 

 is of course the same — the lines of force between the 

 poles are increased in number by the presence of iron ; 

 but whereas, in the first-mentioned mode of treatment, 

 the fact of permeability had to be accepted unexplained, 

 in the second nothing is unexplained except the funda- 

 mental facts of the subject, such as the reason why currents 

 tend to set themselves with their axes parallel, and other 

 matters of that sort. 



Electrical Motuentum once more. 



There is just one point which I must stop here to call 

 attention to. The theories of magnetism and dia- 

 magnetism, which I have given according to Ampere, 

 Weber, and Maxwell, require as their foundation that in a 

 perfect conductor electricity shall obey the first law of 

 motion — shall continue to flow until stopped by force. 

 But the property of matter which enables it to do this is 

 called iiieriia; the law is called the law of inertia ; and 

 anything which behaves in this way must be granted to 

 possess inertia. 



It would not do to deduce so important a fact from a 

 yet unverified theory ; but at least one must notice that 

 it is essentially involved in Ampere's theory of magnetism. 



It is the only theory of magnetism yet formulated, and it 

 breaks down unless electricity possesses inertia. 



Nevertheless it is a fact that an electro- magnet does 

 not behave in the least like a fly-wheel or spinning-top : 

 there is no momentum mechanically discoverable. Sup- 

 posing this should turn out to be strictly and finally true, 

 we must admit that a molecular electric current consists 

 of two equal opposite streams of the two kinds of electri- 

 city : one must begin to regard negative electricity not 

 as merely the negation or defect of positive, but as a 

 separate entity. Its relation to positive may turn out to be 

 something more like that of sodium to chlorine than that 

 of cold to heat. 



I said that no effect due to electric inertia was 

 mecha7iically discoverable, but that is perhaps too sweep- 

 ing a statement. Think of a couple of india-rubber pipes 

 tied together so as to form a double tube, and through 

 each propel a current of water, one in an opposite direc- 

 tion to the other. Although the double current has na 

 gyrostatic properties, yet the water exhibits momentum, 

 even when the current is quite steady, by its effect on 

 kinks and bends and curves in the tube : these all tend 

 to straighten or smooth themselves out, and the tube if 

 quite free would become a perfect circle. 



Precisely the same effect can be observed with a 

 flexible conducting wire or gold thread. Throw a loop of 

 very hght.flexible thinly-covered stranded wire at random 

 on a glass slab, and pass a strong current through it : it 

 will tend to round off its sharp corners, open out its 

 tangled loops, and do its best to become a perfect circle ; 

 and this quite independently of the earth's field, in 

 accordance with the principle numbered 3 on page 8. It 

 will be at once objected that this effect, in the case of the 

 wire, is due to something going on in the medium sur- 

 rounded by it, and not simply to the inertia of anything in 

 the conducting channel itself, as in the water case. The 

 objection is, of course, perfectly valid, but nevertheless 

 the effect is one worth bearing in mind ; and its ultimate 

 explanation may lead us to postulate inertia quite as 

 essentially though not so superficially as the crude 

 hydraulic analogy suggests. 



So long as one considered the flow of electricity in 

 ordinary conductors, we could partially'avoid the question 

 of inertia by considering it urged forward at every point 

 with a force sufficient to overcome the resistance there 

 and no more ; but though this explained the shape of the 

 stream-lines (Fig. 15) yet it did not suffice to render clear 

 the phenomena of self-induction — the lag of the interior 

 electricity in a wire behind the outside until definitely 

 pushed ; and still more its temporary persistence in motion 

 after the pushing force has ceased. 



But, now that we are dealing with perfect conductors 

 with no pushing force at all, the persistence of molecular 

 currents without inertia, or an equivalent property so like 

 it as to be rightly called by the same name at present, 

 becomes inexplicable. True, the molecular currents are 

 as yet an hypothesis ; and that is the only loop-hole out of 

 a definite conclusion. 



Oliver J. Lodge. 

 {To be continued^ 



DISCOVERY OF DIAMONDS IN A METEORIC 

 STONE. 



TN a Russian paper of October 22 last appears a pre- 

 ■'■ liminary report of the examination by Latschinof 

 and Jerofeief, Professors of Mineralogy and Chemistry 

 respectively, of a meteoric stone weighing 4 lbs., which 

 fell in the district of Krasnoslobodsk, Government of 

 Pensa, Russia, on September 4, 1886. 



In the insoluble residue small corpuscles showing 

 traces of polarization were observed ; they are harder 

 than corundum, and have the density and other characters 



