180 



SCIENCE 



[Vol. IT., No. 82. 



scale, and intrusted to the hands of ordinary work- 

 men. 



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 every 

 thing relating to measurement on a small scale ac- 

 quired an importance and development for which we 

 might otherwise have had long to wait, so now the re- 

 quirements 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 resistance-coils on which the elec- 

 trician relies, in dealing with currents whose maxi- 

 mum is a fraction of an ampere, fail altogether when 

 it becomes a question of hundreds, not to say thou- 

 sands, of amperes. 



The powerful currents which are now at com- 

 mand 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 pro- 

 duced at will on the most conspicuous scale. Con- 

 sider, for a moment, Faraday's great discovery of the 

 'magnetization of light,' which Tyndall likens to 

 the Weisshorn among mountains, as high, beautiful, 

 and alone. 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 polariza- 

 tion 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 vapors, where, on account 

 of its extreme smallness, it had previously escaped 

 notice. 



Reference was made to the importance the question 

 of the magnetic saturation of iron was assuming in 

 the discussion of the problems arising in connection 

 with the dynamo-machines, and to the work of Row- 

 land and Stoletow on the theory of the behavior of 

 soft iron under varying magnetic conditions. 



The introduction of powerful alternate-current 

 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 resist- 

 ance. On this principle, much may be explained that 

 would otherwise seem paradoxical. To take a com- 

 paratively simple case, conceive an electro-magnet 

 wound with two contiguous wires, upon which acts 

 a given rapidly periodic electromotive force. If one 

 wire only be used, a certain amount of heat is de- 

 veloped in the circuit. Suppose, now, that the second 

 wire is brought into operation in parallel, — a pro- 



ceeding 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 con- 

 clusion would be entirely erroneous. The total cur- 

 rent, being governed practically by the self-induction 

 of the circuit, would not be augmented by the acces- 

 sion of the second wire ; and the total heating-effect, 

 so far from being doubled, would, in virtue of the 

 superior conductivity, be halved. 



During the last few years, much interest has been 

 felt in the reduction to an absolute standard of meas- 

 urement of electromotive force, current, resistance, 

 etc. ; and to this end many laborious investigations 

 have been undertaken, some of the results being 

 embodied in the resolves of the Conference of elec- 

 tricians assembled at Paris. 



For the measurement of current strength, advan- 

 tage may be taken of Faraday's law, that the quan- 

 tity of metal decomposed in an electrolytic cell is 

 proportional to the whole quantity of electricity that 

 passes. The best metal for the purpose is silver, 

 deposited from a solution of the nitrate or of the 

 chlorate. The results recently obtained by Professor 

 Kohlrausch and by Lord Rayleigh are in very good 

 agreement ; and the conclusion that one ampere, 

 flowing for one hour, decomposes 4.025 grains of sil- 

 ver, can hardly be in error by more than a thousandth 

 part. This number being known, the silver voltame- 

 ter gives a ready and very accurate method of meas- 

 uring currents of intensity, varying from a tenth of 

 an ampere to four or five amperes. 



The beautiful and mysterious phenomena attend- 

 ing the discharge of electricity in nearly vacuous 

 spaces have been investigated and in some degree 

 explained by De la Rue, Crookes, Schuster, Moulton, 

 and the lamented Spottiswoode, as well as by various 

 able foreign experimenters ; and a remarkable obser- 

 vation 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 discussion. 



Without doubt, the most important achievement of 

 the older generation of scientific men has been the 

 establishment and application of the great laws of 

 thermo-dynamics, or, as it is often called, the me- 

 chanical theory of heat. The first law, which asserts 

 that heat and mechanical work can be transformed 

 one into the other at a certain fixed rate, is now 

 well understood by every student of physics; and 

 the number expressing the mechanical equivalent of 

 heat resulting from the experiments of Joule has 

 been confirmed by the researches of others, and espe- 

 cially of Rowland. But the second law, which practi- 

 cally is even more important than the first, is only 

 now beginning to receive the full appreciation due 

 to it. One reason of this may be found in a not 

 unnatural confusion of ideas. Words do not always 

 lend themselves readily to the demands that are made 

 upon them by a growing science; and the almost un- 

 avoidable use of the word 'equivalent' in the state- 



