546 



NATURE 



{Oct. 6, 1 88 1 



beneath the notice of a man with a fair share of intellect and 

 diligence. 



And this opinion was quite justified by the facts. In my own 

 University — London — until quite recently, there was no evidence 

 of practical knowledge required in any branch of science except 

 botany, for the degree of Bachelor of Science. A fair amount 

 of mathematics and mathematical physics were demanded ; but 

 the chemical standard was miserably low, and the zoology, 

 physiology, botany, and geology \\ere such that no experienced 

 examinee would wish for more than a month's reading for each, 

 with perhaps an extra fortnight in the case of botany to enable 

 him to learn enough of the art of describing plants. But now 

 that a searching practical examination is enforced in these 

 subjects, the degree has a real value — it is evidence that a man 

 has done real work. 



The case is very similar at Cambridge. Formerly, the 

 Natural Science Tripos was a bye-word — a sort of bacli-door to 

 a university degree. Now, thanlis in great measure to Dr. 

 Foster, the chances are that a man who takes high honours in 

 that Tripos will be the intellectual equal of a high wrangler or 

 of a high classic. 



Considering that this regeneration of biological teaching began 

 only about ten years ago in London and Cambridge, I think 

 New Zealand is distinctly to be congratulated upon the fact that 

 the first professor of l)iology in the Colony — my predecessor in 

 this Chan-, Captain Hutton — was also the first to inaugurate the 

 true method of teaching that science in the Australian "Colonies. 

 It is by no means the least important debt which the C' Jony 

 owes to Prof. Ilutton, that he, having made his reputation as a 

 systematic zoologist, voluntarily undertook the labour— no lisjht 

 one — of organising, in connection with his lectures, a class for 

 regular practical instruction in comparative anatomy. I must 

 confess to a slight feeling of disa|:'pointment at finding, on my 

 arrival here, that the revolution I had expected to initiate was 

 already well under weiyh. 



( To be ctntitiued. ) 



THE ELECTRICAL DISCHARGE, ITS FORMS 



AND ITS FUNCTIONS'- 



I. 



T F we knew as much about electricity as we know about sound 



or light, we should be still a long way from having learnt 



all that we could wish, but we should know far more than we 



do now. 



For instance, in the matter of sound, we linow, in most cases, 

 the nature of the air disturbance to which it is due, and the 

 mechanism \\herel>y that disturbance is effected ; and we have 

 ascertained the magnitude and character of the aerial waves on 

 which sound is carried. We know, in fact, what it is which is 

 transmitted, and the velocity and direction in which that trans- 

 mission takes place. 



Again, in the matter of light, although we do not know the 

 exact nature of the disturbance to which luminosity is due, nor 

 the mechanical process by whicli that disturbance is effected ; 

 although we are not even certain whether the anherial waves, to 

 which light is attributed, have an actual existence or not. Re 

 nevertheless do know that something w hich is capable of being 

 repre-ented by wave motion is transmitted along a ray of light ; 

 its direction is a matter of simple observation, and we have 

 determined the velocity with which it travels. 



But when we come to electricity our knowledge is much more 

 at fault. We know, it is true, how to produce electricity or 

 electrical action, as well as how to transmit it, by means of 

 wires, to a distance ; we know also that there is a dissymmetry 

 at the two ends or "terminals" of a battery or machine, or other 

 source of electricity, implying a directional character either in 

 that which is transmitted, or in the mode of its transmission. 

 But we know neither what electricity really is, nor the process 

 whereby it is transmitted. And although, on account of the 

 dissymetry above mentioned, we cannot "divest ourselves of the 

 idea of direction, yet we have as yet no certain clue to the actual 

 direction in w hich the transmission can be said to take place. 

 It has, indeed, been shown, by the late Clerk Maxwell and others, 

 that the mathematical expressions for the properties of a medium, 

 whose vibrations are capable of representing the phenomena 

 of light, are the same as those of a medium whose vibra- 

 tions are capable of representing those of electro-mag- 



' A Lecture delivered before the British Association at York on September 

 5, i88i, by William Spottiswoode, D.C.L., LL.D., President of the Royal 

 .Society. 



netism ; and that, on the supposition that light is an electro- 

 magnetic phenomenon, the velocity of propagation of electro- 

 magnetic disturbances is the same as the velocity of light. But 

 an identity in the mode of mathematical representation does not 

 decide anything about the physical facts in either case, nor does 

 It even prove that the facts are the same in both cases. And 

 lastly, even granting that there is actual motion along the wires, 

 neither the mathematical formulze nor the experimental facts 

 can as yet decide whether the motion, or "current" of electri- 

 city, is to be considered as starting from one terminal and 

 arriving at the other, or as starting from the second and arriving 

 at the first ; or, indeed, whether the motion may not be in some 

 sense double, in both directions at once. 



In this somewhat unsatisfactory state of ignorance we approach 

 the subject of this evening's discoure. And although I cannot 

 hope in any adequate sense to resolve the-e difficulties, I pro- 

 pose to explain what progress has been mide towards a solution 

 of them, and to indicate the direction which appears to offer the 

 best promi e of success in the prosecution of further re earch. 



Into the various modes of producing electricity it is not my 

 intention now to enter. I shall use them indifferently as may be 

 mist convenient, explaining only in general terms any differences 

 which may be of consequence for understanding the various 

 ex, eriments shown in illustration of my argument. It will, in 

 fact, be assumed that electricity has been | roduced Ijy some 

 known means or other, and our object will be to examine it in 

 the course i^f its passage, with a view of obtaining some informa- 

 tion as to its nature and its mode of transmission. 



As a matter of fact we have here as our sources of electricity, 

 first, a Holtz machine, or, rather. Prof. Tbppler's modification 

 of it), which produces electricity in a cohdilioii similar to that 

 given off by the ordinary frictional machines, although it effects 

 this by a different method ; secondly, a battery, or arrangement 

 of metallic plates and acid, wherein a flow or "current" of 

 electricity is produced by the action of the acid upon the metal ; 

 thirdly, a dyuamj-mnchine, such as those invented by Gramme, 

 .Siemens, Brush, or others, which produces a current similar to 

 that from the battery, but by means of the ex| enditure of me- 

 chanical force in moving coils or other closed circuits of wire 

 within the influence of an electro-magnet, or, as it is usually 

 termed, within a magnetic field ; fourthly, a magneto-machine 

 by De Meritens, producing, on a j)rinciple similar to that involved 

 in the dynamo-machine, a series of current-, but with permanent 

 magnets, and in this case in alternate directions ; fifthly, an in- 

 strument called an induction-coil, the object of which is to pro- 

 duce from currents of one character currents of another, in a 

 way to be presently described ; and, lastly, we have Leyden jars 

 or condensers for accumulating large charges in a manner which 

 will allow < f their being discharged all at once. 



Now, in the first place, suppose we make use of the battery, 

 or of the dynamo-machine, iroducing a direct and practically 

 uniform current; then, if the wires canying the current be ; 

 closed, no directly visible effect is produced. I say "directly 

 visible " because indirectly we can prove that a wire carrying a 

 current is in a condition different to one not carrying a current. 

 One way in which this may be shown is the following : — If we 

 bring an ordinary jiiece of copper wire into the neighbourhood of 

 STme iron filings, the filings are indifferent to its presence when it 

 i^. in its natural state ; but as soon as the wire is made part of a 

 circuit through which a current is flov\ing, the filings are attracted 

 by it as if by a magnet. When the circuit is broken, so that the 

 current is interrupted, the filings drop, and the wire resumes its 

 ordinary condition. This property of a wire carrying a current 

 is, however, beside our present purpose, and I mention it only ■ 

 in order to show that the passage of an electric current is not "Ij 

 without its effect on a closed circuit, even when no result is 

 directly visible. 



The magnetic effect which we have just seen is not, however, 

 the only effect which a current produces in a closed circuit. If 

 in a galvanic circuit, supposed to consist otherwise of copper 

 wire, we interpose a piece of different metal of a kind called 

 refractory on account of its bad conductive power, such as 

 platinum or iron, or a sufficiently thin piece of the same wire, 

 we shall find that when the current is passing, the interposed 

 wire becomes hot ; and if we increase the strength of the current, 

 or reduce the thickness of the wire — in other words, if we 

 increase the quantity of electricity flowing through the platinum, 

 or diminish the size of the platinum conductor which has to 

 carry it — »e shall find that the temperature is proportionally in- 

 creased. A similar increa-ed temperature will be produced by 



