NA TURE 



193 



THURSDAY, JUNE 29, 1893. 



ELECTRO DYNAMICS. 

 Dynamo Electric Machinery. Fourth Edition. Revised 

 and etilarged. By Silvanus P. Thompson. (London : 

 E. and F. N. Spon, 1892.) 



A COMPARISON of the size of the fourth edition ofthis 

 book with that of the first, which appeared in 1884, 

 suppUes a good illustration of the rate at which the use 

 of dynamo electric machinery, and our knowledge of its 

 laws, have advanced during the past eight years. 

 The 408 pages in 1884 have grown to 832, with a collec- 

 tion of twenty-nine excellent plates in addition at the 

 end of the book, representing another 60 pages ; in fact, 

 the book has now become so portly that it would have 

 been well had the matter been put into two volumes. 



Each of the last three editions has been bulkier than 

 its predecessor, but the increase of size of the second 

 and third represented the simple adding of new material 

 without the pruning away of antique and practically 

 obselete matter which is so necessary in a text- 

 book of a rapidly advancing industry. The last 

 edition, on the contrary, has been rewritten, and 

 the author's well-known capacity for hard work and 

 information collecting has enabled him to produce a 

 treatise which contains the latest knowledge and the 

 existing practice of the dynamo designer and the dynamo 

 constructor of to-day. 



The book opens with a wonderfully complete collection 

 of historical notes, but we fear that the author's love of 

 history has led him to give a little too much credit to the 

 early workers. It is the fashion with some, and especi- 

 ally with those of classical tendencies, to credit the 

 Chinese with the invention of gunpowder, the compass, 

 and a variety of other useful commodities ; to condemn 

 Galileo, Columbus, and Harvey as plagiarists ; and to 

 extol Pliny or Aristotle, or other gentlemen of that some- 

 what remote period, as having foreseen and foretold 

 every scientific principle and device. 



But as these prophecies of the ancients were somewhat 

 marred by their utter unsuggestiveness until the dis- 

 coveries of the moderns had set the historian searching for 

 a meaning which the writers of the prophecies were 

 themselves probably quite ignorant of, we do not regard 

 the trousered investigator as a dealer in second-hand 

 articles. 



Nobody knows better than Dr. Thompson that a know- 

 ledge of the properties of rubbed amber, or the discovery 

 of the loadstone, was not all that was necessary to con- 

 struct a 1000 horse-power dynamo with a commercial 

 efficiency of 93 per cent., but nevertheless he fails, we 

 think, to sufficiently distinguish between a chance mention 

 of some notion and the subsequent independent recog- 

 nition of an important commercial principle. If fame 

 could really be achieved by a person's mingling a grain 

 of wheat with a ton of chaff, what a temptation it would 

 be to spend one's time recording every notion that struck 

 one (no matter how improbable it looked), in the hopes 

 that a hundred years hence some indulgent historian 

 would search through the weary waste, in the hope of 

 discovering with his rosy spectacles an apparent anticipa- 

 NO. 1235, VOL. 48] 



tion of some device that practice had then brought to 

 a successful issue. 



The historical notes are in fact not critical enough, and 

 show a desire to make things comfortable all round for 

 everybody. For example, the conventional illustration of 

 the Pacinotti machine is given, but the author does not 

 point out, indeed we do not remember to have seen it 

 pointed out, that the original illustration of the Pacinotti 

 generator differed from the conventional illustration 

 in that the collecting-brushes were placed in the worst 

 position, so as to make the machine as powerless as pos- 

 sible. May this have been the real reason why this 

 machine " fell into temporary oblivion " ? 



If another example were wanted, we might take the 

 following sentence, which, although not occurring in the 

 section called " Historical Notes," enters as a note of an 

 historical character on page 59, in the section, " Com- 

 binations to give Constant Pressure." The sentence is, 

 " The combination of a permanent magnet with electro- 

 magnets in one and the same machine is much older than 

 the suggestions of either Deprez or Perry, having been 

 described by Hjorth in 1854." Undoubtedly that is true, 

 only Hjorth used the combination because, not being 

 aware of the instability of the magnetism in a properly 

 designed dynamo, he thought permanent magnetism was 

 necessary to start the magnetic excitation ; whereas 

 Deprez and Perry superimposed what was practically 

 a permanent field jfor a totally different reason and in a 

 totally different way. 



The chapters on " Magnetic Principles," the " Magnetic 

 Circuit," " Forms of Field Magnets," are excellent. We 

 do not, however, see much advantage in the introduction of 

 what the author calls the diacritical current. The formulas 

 are thereby simplified, no doubt, but the simplification is 

 effected at the sacrifice of accuracy ; for first, the per- 

 manent magnetism in the field has to be ignored, and 

 secondly, as there is no absolute maximum induction in 

 iron, there can be no exact value of this diacritical c\irxtni, 

 which produces half the maximum induction. 



The author is not quite happy in his choice of symbols. 

 E is defined as representing the entire electro-motive force 

 in the armature, e as the difference of potentials 

 from terminal to terminal. Presumably, then, e^, e^, &c., 

 applied to the separate coils of the armature, 

 represent the potential differences at the terminals 

 of the several coils. But that is exactly what e^, e^ do 

 not mean, for they stand for the electro-motive forces of 

 the coils. The suffix m attached to the letter for current 

 or resistance denotes field magnet coils, but only when 

 these are series coils. If the coils be shunt coils, the 

 suffix s is attached to the letter. S, however, stands not 

 for the number of shunt coils, as one would expect, but for 

 the number of series coils, the former being called by a 

 different letter altogether, viz., Z. In fact, Dr. Thomp- 

 son's rules for the use of suffixes have the precision that 

 is possessed by the rules for the spelling of the English 

 language, the delight of every foreigner who studies 



them. 



The description of "Combinations to give Constant 

 Pressure " (pages 57, &c.), and of " Constant Potential 

 Dynamos" (pages 277, &c.), might be well brought together, 

 seeing that both parts of the book deal with the same 

 contrivances, only a little mathematics is added when 



K 



