412 



NA TURE 



{August 28, 1884 



management and the presence on the spot of skilled engineers 

 obviate some of the difficulties that are met with under other 

 circumstances. At present we have no experience of a house-to- 

 house system of illumination on a great scale and in competition 

 with cheap gas ; but preparations are already far advanced for 

 trial on an adequate scale in London. In large institutions, such 

 as theatres and factories, we all know that electricity is in 

 successful and daily extending operation. 



When the necessary power can be obtained from the fall of 

 water, instead of from the combustion of coal, the conditions of 

 the problem are far more favourable. Possibly the severity of 

 your winters may prove an obstacle, but it is impossible to regard 

 your splendid river without the thought arising that the day may 

 cone when the vast powers now running to waste shall be bent 

 into your service. Such a project demands of course the most 

 careful consideration, but it is one worthy of an intelligent and 

 enterprising community. 



The requirements of practice react in the most healthy manner 

 upon scientific electricity. Just as in former days the science 

 received a stimulus from the application to telegraphy, under 

 which everything relating to measurement on a small scale 

 acquired an importance and development for which we might 

 otherwise have had long to wait, so now the requirements of 

 electric lighting are giving rise to a new development of the art 

 of measurement upon a large scale, which cannot fail to prove of 

 scientific as well as practical importance. Mere change of 

 scale may not at first appear a very important matter, but it is 

 surprising how much modification it entails in the instruments, 

 and in the processes of measurement. For instance, the resist- 

 ance coils on which the electrician relies in dealing with currents 

 whose maximum is a fraction of an ampere fail altogether when 

 it becomes a question of hundreds, not to say thousands, of 

 amperes. 



The powerful currents, which are now at command, constitute 

 almost a new weapon in the hands of the physicist. Effects 

 which in old days were rare and difficult of observation may 

 now be produced at will on the most conspicuous scale. Con- 

 sider for a moment Faraday's great discovery of the " Magneti- 

 sation of Light," which Tyndall likens to the Weisshorn among 

 mountains, as high, beautiful, and alone. This judgment (in 

 which I fully concur) relates to the scientific aspect of the dis- 

 covery, for to the eye of sense nothing could have been more 

 insignificant. It is even possible that it might have eluded 

 altogether the penetration of Faraday, had he not been provided 

 with a special quality of very heavy glass. At the present day 

 these effects may be produced upon a scale that would have 

 delighted their discoverer, a rotation of the plane of polarisation 

 through 180° being perfectly feasible. With the aid of modern 

 appliances, Kundt and Rontgen in Germany, and H. Becquerel 

 in France, have detected the rotation in gases and vapours, 

 where, on account of its extreme smallness, it had previously 

 escaped notice. 



Again, the question of the magnetic saturation of iron has now 

 an importance entirely beyond what it possessed at the time of 

 Joule's early observations. Then it required special arrange- 

 ments purposely contrived to bring it into prominence. Now in 

 every dynamo machine, the iron of the field-magnets approaches 

 a state of saturation, and the very elements of an explanation of 

 the action require us to take the fact into account. It is indeed 

 probable that a better knowledge of this subject might lead to 

 improvements in the design of these machines. 



Notwithstanding the important work of Rowland and Stoletow, 

 the whole theory of the behavi uir of soft iron under varying 

 magnetic conditions is still somewhat obscure. Much may be 

 hoped from the induction balance of Hughes, by which the 

 marvellous powers of the telephone are applied to the discrimina- 

 tion of the properties of metals, as regards magnetism and 

 electric conductivity. 



The introduction of powerful alternate-current in machines by 

 Siemens, Gordon, Ferranti, and others, is likely also to have a 

 salutary effect in educating those so-called practical electricians 

 whose ideas do not easily rise above ohms and volts. It has long 

 been known that when the changes are sufficiently rapid, the 

 phenomena are governed much more by induction, or electric 

 inertia, than by mere resistance. On this principle much may 

 be explained that would otherwise seem paradoxical. To take 

 a comparatively simple case, conceive an electro-magnet wound 

 with twi contiguous wires, upon which acts a given rapidly 

 periodic electromotive force. If one wire only be used, a certain 

 amount of heat is developed in the circuit. Suppose now that 



the second wire is brought into operation in parallel — a proceed- 

 ing equivalent to doubling the section of the original wire. An 

 electrician accustomed only to constant currents would be sure 

 to think that the heating effect would be doubled by the change, 

 as much heat being developed in each wire separately as was at 

 first in the single wire. But such a conclusion would be entirely 

 erroneous. The total current, being governed practically by 

 the self-induction of the circuit, would not be augmented by the 

 accession of the second wire, and the total heating effect, so far 

 from being doubled, would, in virtue of the superior conduc- 

 tivity, be halved. 



During the last few years much interest has been felt in the 

 reduction to an absolute standard of measurements of electro- 

 motive force, current, resistance, &c, and to this end many- 

 laborious investigations have been undertaken. The subject i> 

 one that has engaged a good deal of my own attention, and I 

 should naturally have felt inclined to dilate upon it, but that I 

 feel it to be too abstruse and special to be dealt with in detail 

 upon an occasion like the present. As regards resistance, I will 

 merely remind you that the recent determinations have shown a 

 so greatly improved agreement that the Conference of Electri- 

 cians assembled at Paris in May have felt themselves justified in 

 defining the ohm for practical use as the resistance of a column 

 of mercury of 0° C, one square millimetre in section, and 106 

 cm. in length — a definition differing by a little more than I per 

 cent, from that arrived at twenty years ago by a committee of 

 this Association. 



A standard of resistance once determined upon can be em- 

 bodied in a " resistance coil," and cipied without much trouble, 

 and with great accuracy. But in order to complete the electrical 

 system, a second standard of some kind is necessary, and this is 

 not so easily embodied in a permanent form. It might conve- 

 niently consist of a standard galvanic cell, capable of being pre- 

 pared in a definite manner, whose electromotive force is once for 

 all determined. Unfortunately, most of the batteries in ordinary 

 use are for one reason or another unsuitable for this purpose, 

 but the cell introduced by Mr. Latimer Clark, in which the 

 metals are zinc in contact with saturated zinc sulphate and pure 

 mercury in contact with mercurous sulphate, appears to give 

 satisfactory results. According to my measurements, the electro- 

 motive force of this cell is i'435 theoretical volts. 



We may also conveniently express the second absolute electri- 

 cal measurement necessary to the completion of the system by 

 taking advantage of Faraday's law that the quantity of metal 

 decomposed in an electrolytic cell is proportional to the whole 

 quantity of electricity that passes. The best metal for the pur- 

 pose is silver, deposited from a solution of the nitrate or of the 

 chlorate. The results recently obtained by Prof. Kohlrausch 

 and by myself are in very good agreement, and the conclusion 

 that one ampere flowing for one hour decomposes 4*025 grains 

 of silver, can hardly be in error by more than a thousandth part. 

 This number being known, the silver voltameter gives a ready 

 and very accurate method of measuring currents of intensity 

 varying from one-tenth of an ampere to four or five amperes. 



The beautiful and mysterious phenomena attending the dis- 

 charge of electricity in nearly vacuous spaces have been investi- 

 gated and in some degree explained by De La Rue, Crookes, 

 Schuster, Moulton, and the lamented Spottiswoode, as well as 

 by various able foreign experimenters. In a recent research 

 Crookes has sought the origin of a bright citron-coloured band 

 in the phosphorescent spectrum of certain earths, and after 

 encountering difficulties and anomalies of a most bewildering 

 kind, has succeeded in proving that it is due to yttrium, an ele- 

 ment much more widely distributed than had been supposed. A 

 conclusion like this is stated in a few words, but those only who 

 have undergone similar experience are likely to appreciate the 

 skill and perseverance of which it is the final reward. 



A remarkable observation by Hall of Baltimore, from which 

 it appeared that the flow of electricity in a conducting sheet was 

 disturbed by magnetic force, has been the subject of much dis- 

 cussion. Mr. Shelford Bidwell has brought forward experiments 

 tending to prove that the effect is of a secondary character, due 

 in the first instance to the mechanical force operating upon the 

 conductor of an electric current when situated in a powerful 

 magnetic field. Mr. Bidwell's view agrees in the main with Mr. 

 Hall's division of the metals into two groups according to the 

 direction of the eff ct. 



Without doubt the most important achievement of the older 

 generation of scientific men has been the establishment and 



