NATURE 



{Nov. 3, 1887 



CH2OH. Both theories account for most of the hitherto 

 known reactions of the glucoses, hence the matter has 

 remained an open question, Drs. Fischer and Tafel, 

 however, consider that their synthesis from acrolein, 

 which is itself an aldehyde, points to the probability of 

 the former hypothesis being the correct one. The action 

 of baryta water upon the dibromide evidently causes a 

 simple exchange of bromine for hydroxyl, and the first 

 product of the reaction is almost as certainly glycerine 

 aldehyde, CH2OH— CHOH— CHO. This latter sub- 

 stance, however, appears to polymerize at once under 

 the influence of the baryta water into sugar, two mole- 

 cules of glycerine aldehyde uniting to form a molecule of 

 glucose. 



In consideration of the fact of its derivation from 

 acrolein, the name acrose has been applied to the sugar 

 which has been, with so much skill and steady determi- 

 nation, synthetically formed and isolated ; and there can 

 be no doubt that this name will stand as a memento of 

 the progress made in organic chemistry during the year 

 1887. A. E. TUTTON. 



MODERN VIEWS OF ELECTRICITY} 

 PART II.— Current Electricity {continued). 



IV. 



Electrical Inertia. 



"D ETURNING now to the general case of conduction, 

 -*-^ without regard to the special manner of it, we must 

 notice that, if a current of electricity is anything of the 

 nature of a material flow, there would probably be a certain 

 amount of inertia connected with it, so that to start a 

 current^with a finite force would take a little time ; and 

 the stoppage of a current would also have either to 

 be gradual or else violent. It is well known that if 

 water is stagnant in a pipe it cannot be quite suddenly 

 set in motion ; and again, if it be in motion, it 

 can only be suddenly stopped by the exercise of very con- 

 siderable force, which jars and sometimes bursts the 

 pipe. This impetus of running water is utilized in the 

 water-ram. It must naturally occur, therefore, to ask 

 whether any analogous phenomena are experienced with 

 electricity ; and the answer is, they certainly are. A 

 current does not start instantaneously : it takes a certain 

 time — often very short — to rise to its full strength ; and 

 when started it tends to persist, so that if its circuit be 

 suddenly broken, it refuses to stop quite suddenly, and 

 bursts through the introduced insulating partition with 

 violence and heat. It is this ram or impetus of the 

 electric current which causes the spark seen on breaking 

 a circuit ; and the more sudden the breakage the more 

 violent is the spark apt to be. 



The two effects — the delay at making circuit, and the 

 momentum at breaking circuit — used to be called " extra- 

 current" effects, but they are now more commonly spoken 

 of as manifestations of " self-induction." 



We shall understand them better directly ; meanwhile 

 they appear to be direct consequences of the inertia of 

 electricity ; and certainly if electricity were a fluid pos- 

 sessing inertia it would behave to a superficial observer 

 just in this way. 



But if an electric current really possessed inertia, as a 

 stream of water does, it would exhibit itself not only by 

 these effects but also mechanically. A conducting coil 

 delicately suspended might experience a rotary kick every 

 time a current was started or stopped in it ; and if a steady 

 current were maintained in such a coil it should behave 

 like a top or gyrostat, and resist any force tending to 

 deflect its plane. 



Clerk Maxwell has carefully looked for this latter form 

 of momentum effect, and found none. One may say, in 

 fact, that nothing like momentum has yet been observed 



' Con.inued from vol. xxxvi. p. 585. 



in an electric current by any mechanical mode of examina- 

 tion. A coil or whirl of electricity does not behave in the 

 least like a top. 



Does this prove that a current has no momentum ? By 

 no means necessarily so. It might be taken as suggesting 

 that an electric current consists really of two equal flows 

 in contrary directions, so that mechanically they neutralize 

 one another completely, while electrically — i.e. in the 

 phenomena of self-induction or extra-current — they add 

 their effects. Or it may mean merely that the momentum 

 is too minute to be so observed. Or, again, the whole 

 thing — the appearance of inertia in some experiments and 

 the absence of it in others — may have to be explained in 

 some altogether less simple manner, to which we will 

 proceed to lead up. 



Condition of the Medium near a Circuit. 



So far we have considered the flow of electricity as a 

 phenomenon occurring solely inside conductors ; just as 

 the flow of water is a phenomenon occurring solely inside 

 pipes. But a number of remarkable facts are known 

 which completely negative this view of the matter. 

 Something is no doubt passing along conductors when a 

 current flows, but the disturbance is not C07ijined to the 

 conductor ; on the contrary, it spreads more or less 

 through all surrounding space. 



The facts which prove this have necessarily no 

 hydraulic analogue but must be treated suorum generum, 

 and they are as follows : — 



(i) A compass needle anywhere near an electric 

 current is permanently deflected so long as the current 

 lasts. 



(2) Two electric currents attract or repel one another, 

 according as they are in the same or opposite directions. 



(3) A circuit in which a current is flowing tends to 

 enlarge itself so as to inclose the greatest possible area. 



(4) A circuit conveying a current in a magnetic field 

 tends either to enlarge or to shrink or to turn half round 

 according to the aspect it presents to the field. 



(5) Conductors in the neighbourhood of an electric 

 circuit experience momentary electric disturbances every 

 time the current is started or stopped or varied in strength. 



(6) The same thing happens even with a steady current 

 if the distance between it and a conductor is made to 

 vary. 



(7) The effects of self-induction, or extra-currents, can 

 be almost abolished by twisting the covered wire convey- 

 ing the current closely on itself, or even by laying the 

 direct and return wire side by side ; whereas they may be 

 intensified by making the circuit inclose a large area, more 

 by coiling it up tightly into close coil, and still more by 

 putting a piece of iron inside the coil so formed. 



Nothing like any of these effects is observable with 

 currents of water ; and they prove that the phenomena of 

 the current, so far from being confined to the wire, 

 spread out into space and affect bodies at a considerable 

 distance. 



Nearly all this class of phenomena were discovered by 

 Ampere and by Faraday, and were called by the latter 

 " current-induction." According to his view the dielectric 

 medium round a conducting circuit is strained, and 

 subject to stresses, just as is the same medium round an 

 electrically charged body. The one is called an electro- 

 static strain, the other an electro-magnetic or electro- 

 kinetic strain. 



But whereas electrostatic phenomena occur solely in 

 the medium — conductors being mere breaks in it, inter- 

 rupters of its continuity, at whose surface charge-effects 

 occur but whose substance is completely screened from 

 disturbance — that is not the case with electro-kinetic 

 phenomena. It would be just as erroneous to conceive 

 electro-kinetic phenomena as occurring solely in the insu- 

 lating medium as it would be to think of them as occurring ; 

 solely in the conducting wires. The fact is, they occur in 



