NATURE 



489 



THURSDAY, SEPTEMBER 24, 1891. 



PHYSICAL UNITS AND CONSTANTS. 

 Illustrations of the C.G.S. System of Units ^ with Tables 

 of Physical Constants. By Prof. Everett, F.R.S. 

 (London : Macmillan and Co., 1891.) 



THIS may be taken to be the fourth edition of a work 

 first published by the Physical Society in a some- 

 what different form. Those who know Dr. Everett need 

 not be told that he has done everything that it is possible 

 for an accurate, painstaking author to do, to bring each 

 successive edition as near to perfection as possible. The 

 value of the work to the physics investigator is exceed- 

 ingly great, as everybody knows ; but it is not so generally 

 well known that it is an excellent class exercise book for 

 students. There is much new matter in this edition, in- 

 cluding determinations of viscosities, terrestrial magnetic 

 elements, magnetic properties of iron and other sub- 

 stances, and heat measurements. 



The labours of many men have given to the present 

 generation this beautiful system of units, which has made 

 physical calculation so easy, and which has pointed out 

 in certain cases the directions in which new discoveries 

 might be expected. And it only requires a short study of 

 certain parts of this book to put any student in such 

 possession of the system that he can use it with certainty 

 and ease. Indeed, to become well acquainted with the 

 scientific method of calculation has almost been made 

 too easy for certain clever men of our acquaintance. It 

 is far nobler to swim the Hellespont than to cross in a 

 steamer. At the present time many clever men are 

 possessed by a mania for crossing the Atlantic in boats 

 of eighteen feet keel. It adds much more to one's credit 

 to talk of all kinds of hybrid and home-made magnetic 

 influences, than to use the simple idea of self-induction. 

 In the same way it is unfair to say that certain practical 

 engineers shirk the study of Dr. Everett's book ; it is 

 much better to put it that these gentlemen have too much 

 originality to follow the easy path, and when in their 

 practical applications of physical principles they adopt 

 all sorts of ingenious units of their own manufacture to 

 whose use there are limits in all sorts of ways, we can 

 even feel sorrowful over their skilfulness, without attempt- 

 to thwart their ambition. 



The mechanical engineer is accustomed to the use of a 

 curious unscientific want of system in his calculations. 

 His unit of force is the weight of a pound in London. 

 His velocity is in feet per second, perhaps, in the very 

 same calculation as that in which his pressure is in 

 pounds per square inch. It seems to be too late to 

 change this. No engineer can venture to educate his 

 pupils in the use of the C.G.S. system for mechanical 

 engineering calculations. Mrs. Ali Baba measured her 

 gold by the quart, and a mechanical engineer thinks 

 and designs and talks with other engineers in the usual 

 shop units ; and we may as well think of altering our 

 decimal system to a duodecimal one, as to talk of an 

 alteration in the mechanical engineer's methods of cal- 

 culation. It is a very great pity, but the difficulties in 

 the way of reform seem to be insurmountable. The 

 story of these difficulties is too long for the present 

 NO. I 143, VOL. 44] 



notice. But in new applications of physics, in electrical 

 engineering, for example, the use of the C.G.S. system is 

 not only easy ; it requires a large amount of ingenuity in 

 any engineer to calculate in any other than in C.G.S. 

 units, unless, indeed, he ignores all the experimental de- 

 terminations already made for him and tabulated in the 

 C.G.S. system. And yet such ingenuity has already been 

 exercised, and laborious investigations have been carried 

 out by some electrical engineers, with the result that 

 certain parts of electrical engineering are getting to be 

 even more unscientific in the units employed than any 

 part of mechanical engineering. On behalf of the cul- 

 prits we may say, however, that even Dr. Everett's book 

 — their best guide — has not given them the precise in- 

 formation that it might have done. In the subject of 

 heat, we can now ignore the steam-engine constructor ; 

 we can say to him, " Go on using your wretched pounds 

 per square inch and your foot-pounds per minute, and 

 we will go on using our dynes^per square centimetre and 

 our ergs per second because we are nearly independent 

 of one another " ; but we can make no such speech to the 

 electrical engineer. We physicists have to say to him 

 that we rely upon him to make new discoveries, to state 

 to us new problems ; and if he gives us information in 

 vague units of his own, we cannot tabulate it for general 

 use, and if he does not state to us his problem in the 

 usual language, we are unable to understand him, and 

 we can be of no mutual use to each other. But when he 

 says to us that our language is cumbrous, that he has ideas 

 to express for which we have no words, when he uses 

 towards us, properly for once, that adjective " academic " 

 which has been more misused than Shakespeare's word 

 " occupy," the culprit and the judge change places. 



We can blame him if he invents unsystematic units, 

 but not until we have given him the language and units 

 that are correct. And in some particulars the electric 

 engineer has the right to blame us. For example, our 

 definition of unit electric current is so stupid that a mul- 

 tiplier or divisor of tt or \tt enters quite unnecessarily 

 into all electro-magnetic calculation. 



Concerning electro-magnets and the magnetic circuit of 

 a dynamo machine or a transformer, the practical en- 

 gineer has a simple and quite modern way of considering 

 problems, not yet recognized in such orthodox books as 

 this of Dr. Everett. Magneto-motive force and the 

 magnetic resistance of a circuit are expressions which 

 cannot be found in such a book, and it is not at all un- 

 usual for the orthodox physicist to treat the idea under- 

 lying the use of such expressions with profound con- 

 tempt. The engineer and experimenter care less than 

 nothing for " magnetic susceptibility " or for " intensity 

 of magnetization," or for " free magnetism " ; these are, 

 to him, mementos of the time of twelve years ago, when 

 the inventor made bricks in Egypt, and the very cleverest 

 mathematical electricians were only distinguished from 

 other inventors by the greater magnitude of their blun- 

 der.<;. Dr. Hopkinson and Mr. Kapp and Mr. Bosanquet 

 have given us simple ways of dealing with practical 

 problems, and some of these are now known to every 

 apprentice of an electric engineering factory ; but we 

 know of no mathematical treatise in which they are re- 

 cognized. Is it too much to hope that Dr. Everett, in 

 his next edition, will ignore the orthodox critics, and 



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