41 



NATURE 



[September 3, il 



and its adoption now is probably only an anticipation of 

 the decision which will be arrived at in a i&^N years when 

 the resolutions of that Conference are reconsidered. 



In the adoption of a metallic working standard (an- 

 nounced in Resolution 5) the Committee only endorse an 

 opinion long ago expressed by working electricians, that 

 the mercury standards constructed in straight or spiral 

 glass tubes are not practical instruments ; they are diffi- 

 cult to handle, liable to breakage, and the only argument 

 for their retention, the possible variability of metallic 

 standards, has been shown to be almost baseless by the 

 results of the continued and careful observation of the 

 various metallic resistance coils deposited at Cambridge. 



Passing over the resolutions which provide for copies, 

 and multiples and submultiples of the ohm, with the 

 remark that the long-felt want of trustworthy standards of 

 low resistance will now at last be supplied, we come to the 

 definition of the unit of current. Here again a theoretical 

 definition corresponding to that of the ohm is given first ; 

 then for practical purposes it is stated "that an unvarying 

 current which, when passed through a solution of nitrate 

 of silver in water, in accordance with the specification 

 attached to this report, deposits silver at the rate of 

 0001 1 18 of a gramme per second, may be taken as a 

 current of i ampere." This is the most reasonable course 

 that could have been adopted. The specification is prac- 

 tically one of the procedure adopted by Lord Rayleigh in 

 his experiments on the electro-chemical equivalent of 

 silver, and as Lord Rayleigh's absolute result was to be 

 made the practical standard, it was right to recommend 

 the same mode of experimenting. 



Resolution 11, which defines the ampere in the case of 

 an alternating current, was the subject of a good deal of 

 discussion, and of some adverse comment by one of the 

 witnesses examined on behalf of the electrical trades. 

 The resolution states " that an alternating current of i 

 ampere shall mean a current such that the square root of 

 of the time-average of the square of its strength at each 

 instant in amperes is unity." It was objected by the 

 witness referred to, and by at least one member of the 

 Committee, that this was giving a very special meaning 

 to the term, which was inconsistent with the obvious 

 definition, that of the simple time-average of the current. 

 This latter average would, in the case of most periodic 

 machines, be simply zero, unless the currents in the 

 alternate half-periods were commutated so as to agree in 

 sign with those in the other h.lves. But in the case of 

 such a machine as the Brush, used for lighting incandes- 

 cent lamps, the definition given in the resolution would 

 have to be used ; whereas if the machine were used for 

 electro-plating, the simple time-average would have to be 

 employed. This would give for the same current passing 

 through the machine, from instant to instant, two differ- 

 ent average values. The electric lighting application of 

 periodic machines is, however, by far the most important, 

 and the Committee did well, perhaps, to retain what is 

 already the generally understood sense of the word 

 ampere in connection with alternating currents. It ought 

 to be, however, clearly understood that the main applica- 

 tion of the definition will be to the measurements of cur- 

 rents in electric lighting, and that generally in other cases 

 another definition will have to be employed. 



Another important discussion took place over the 

 NO. I 140, VOL. 44] 



definition of the standard unit of " pressure." In the 

 first place, we should like to say here that we object 

 entirely to the use of the term "pressure" in this connec- 

 tion. It has come as a sort of analogue of hydraulic 

 pressure, and it has certainly led to very erroneous 

 notions in the minds of the general public as to the func- 

 tions of electric supply-mains, and also as to electricity 

 itself. It is a pity that so many of the present pioneers 

 of electricity, who are also leaders of physical science, 

 should have countenanced by their example this misuse 

 of a scientific term. We all know how strenuously some 

 of these gentlemen have objected to the term "tension" as 

 in " high-tension electricity " ; surely " high-pressure in- 

 struments " and " electricity supplied at high pressure " 

 are as objectionable, if not even more misleading. The 

 use of the term voltage, or some such word, in the present 

 Report, would have avoided the endorsement which it 

 seems to give to what we think is a most unfortunate 

 name for a physical quantity which is not a pressure at 

 all ; and it is to be hoped that the British Association 

 Committee (who, by the way, were represented on the 

 Committee of the Board of Trade) may be able to pre- 

 vent this phrase from being added to the many other, 

 though generally less objectionable terms which infest 

 the literature of electricity. 



A discussion arose as to whether the definition of the 

 volt as the " pressure which, if steadily applied to a con- 

 ductor whose resistance is one ohm, will produce a 

 current of one ampere," was sufficiently definite. There 

 might, it was argued, be an internal electromotive force 

 in. the conductor, and the " pressure" applied to the con- 

 ductor might be regarded as that applied from the outside, 

 or actually existent between its terminals, as shown by 

 an electrometer. For example, the conductor might be 

 the armature of a dynamo ; the difference of potential 

 might be considerable and the resistance only a small 

 fraction of an ohm. In such a case it is, of course, well 

 known that the electromotive force producing the current 

 through any part of the armature resistance, according to 

 Ohm's law, is the total internal electromotive force of 

 that part, minus the difference of potential existing 

 between its terminals, and it is this difference that is to 

 be regarded as the applied "pressure" of the definition. 

 In the same way in a voltameter, the electromotive force 

 causing the current, according to Ohm's law, would be 

 the existent or applied difference of potential, minus 

 the internal back electromotive force developed by the 

 chemical action. There were other difficulties about the 

 specification of the ends of the conductor and the canaliz- 

 ation of the current, and it was therefore thought desirable 

 to adhere to the simple form of definition given in the 

 report. It must be admitted that the definition leaves 

 room for legal disputes in practice, and we think that it 

 would have been perhaps better to have introduced on 

 these points some kind of note or specification referred to 

 in the resolution, so as to be taken along with it in the 

 event of any dispute about the meaning of the definition. 



A further question arose as to the provision of a prac- 

 tical standard of electromotive force in the form of a 

 constant cell ; and it was decided, partly in deference to 

 the expressed wish of practical electricians, that the 

 Clark cell should be adopted for this purpose. Its electro- 

 motive force, within certain limits of error to be deter- 



