July 13, 1893] 



NA TURE 



245 



representing the metallic resistance R in circuit, then 

 draw the tangent parallel to this line, and observe 

 whether the ordinate corresponding to the normal work- 

 ing current of the lamp falls on the right or on the left of 

 the point of contact. If E denote the length of the 

 ordinate in question, and C denote the current, we have 

 in the former case 



dE < RdC 

 and in the latter 



dE > RdC 



Thus in the former case the value of dE is smaller 

 than the increment of electromotive force required to 

 drive the corresponding increase of current through the 

 metallic resistance, in the latter case it is larger than 

 this. Consequently, in the latter case, there will be an 

 excess of electromotive force which will go to increase 

 the length of arc. Thus the arc will continually lengthen 

 until the current suddenly fails and the light goes out. 



Hence the mere inspection of the curve settles the 

 question as to whether the machine is running fast 

 enough, or whether there is a sufficient margin of speed 

 to ensure stability. 



In the paper on Some Points in Electric Lighting, a 

 large number of facts, now so well known as to have be- 

 come common places of practical science, are discussed. 

 Hut ten years ago, when the paper was read, many elec- 

 tricians engaged in supplying electric light were them- 

 selves working very much in the dark ; and Dr. Hopkin- 

 son's paper was to many of them exceedingly useful as 

 supplying facts, and especially hints as to graphical pro- 

 cesses of investigating the behaviour of dynamos, whether 

 used as a generator or a motor. 



The next paper is that by the author and his brother, 

 Dr. E. Hopkinson, on Dynamo-Electric Machinery, 

 which has become justly famous as that in which the 

 enormously useful idea of the magnetic circuit was first 

 applied in a complete and consistent manner to the dis- 

 cussion of the results of experiment on different types of 

 dynamo. In this a comparison between the characteristic 

 curve of the machine, and the curve of magnetising force 

 and magnetic induction, is made to give important infor- 

 mation as to the proper disposition of the magnetic 

 circuit, and the failure of the total induction to pass 

 through the armature. Further, the effect of the lead of 

 the brushes and of the current in the armature is fully 

 ussed and graphically illustrated ; and the paper closes 

 iih what were most valuable at the time, a descrip- 

 tion of the author's method of testing the efficiency of 

 dynamos, and numerous results of experiments on 

 machines with armatures wound according to the Hefner 

 Alteneck plan, and the unsymmetrical horseshoe arrange- 

 ment of magnets, and on others with Gramme 

 armatures and the Siemens rectangular symmetrical 

 arrangement of magnets. In these efficiency experiments 

 the ingenious plan of using two similar machines in the 

 same circuit and having their shafts coupled, one acting 

 as generator, the other as motor, was first adopted. 

 The motor in great measure drives the generator which 

 feeds it, and it is only necessary to supply by means of a 

 belt the balance of driving power required. Thus, 

 uncertainty in dynamometric measurement of power 

 transmitted has effect only on the estimation of this 

 balance. The power developed by the motor can be found 

 NO. 1237, VOL. 48] 



electrically, as likewise the electrical energy developed by 

 the generator, and thus all the data are obtained for 

 estimating the efficiency of the machine. 



This idea has borne important fruit in the extremely 

 valuable methods which have been invented by others 

 for more conveniently carrying out similar dynamo ex- 

 periments, and for testing transformers. 



Next comes the very valuable continuation of this 

 paper published only last year, which completes the dis- 

 cussion of direct current machines. In this sequel the 

 effect on the electromotive force of the machine of the 

 current in the armature for a given lead of the brushes is 

 experimentally investigated, and compared with its theo- 

 retical value as given in the earlier part of the paper. 



The remaining portion of the book consists mainly of 

 papers relating to the theory of alternating currents, and 

 the testing of alternate-current machines and trans- 

 formers, and concludes with an account of the author's 

 arrangements for applying the electric light to the light- 

 houses of Macquarie and Tino. The first paper on alter- 

 nating currents is the one which has been so much referred 

 to in recent discussions on the action of alternators, and the 

 possibility of running more than one in the same circuit. 



Though the increased use of alternating currents 

 has added much to our knowledge of the behaviour 

 and capabilities of alternators. Dr. Hopkinson's paper 

 is, and will remain, one of the classics of the sub- 

 ject. But the last word of theoretical and practical 

 explanation has not yet been said, and will probably not 

 be said for a long time. In the meantime there is a 

 possibility, now that the behaviour of iron in rapid mag- 

 netic cycles can be studied completely in various ways, of 

 our obtaining further information which may clear up 

 some of the outstanding difficulties of the subject. 



Some results of a very interesting character as to rapid 

 cycles are given in the paper on the Tests of Westinghouse 

 Transformers. The curves showing the electromotive 

 force and the current at different instants during a half 

 period are plotted and come out very considerably 

 different from the ordinarily assumed curve of series. 

 The harmonic analysis, or the new analysing machine of 

 Henrici and Sharpe, might with advantage be applied to 

 them to reveal their components. From these curves the 

 dissipation loops are plotted and made to give the loss of 

 energy due to local currents and hysteresis in the curves. 



Further description of these papers is unnecessary. 

 They have already passed to a considerable extent into 

 electrical literature ; but a great service to practical elec- 

 tricians has nevertheless been done by their publication 

 in a collected form. 



In Messrs. Hawkins and Wallis's book we have little 

 of originality ; but what seems a straightforward, 

 accurate, and fairly full account of dynamo-electric 

 machinery. Beginning with chapters on the Magnetic 

 Field, the Magnetic Circuit, the Production of an 

 E. M. F., and Self-induction, the authors enter on 

 their main subject with a chapter on the Classifica- 

 tion of Dynamos. The principal types of machine 

 are described and well illustrated, so far as the 

 number and general nature of the cuts are concerned. 

 But while the authors have been liberal with carefully 

 made drawings and well considered diagrams, the 

 execution and printing of the illustrations in the text 



