262 



NATURE 



[June 6, 191 8 



writer's opinion it forms a very beautiful adden- 

 dum to Prof. Jaeger's principle, especially as 

 these regular differences become smaller as the 

 symmetry becomes higher, until in such series of 

 relatively high symmetry as the hexagonal ethyl 

 sulphates of the rare earths investigated by Prof. 

 Jaeger they are reduced almost to the limits of 

 experimental error, while in the cubic system they 

 disappear altogether. One statement made by the 

 author can scarcely be accepted, that isomorphism 

 is only a special case of morphotropy. For in an 

 isomorphous series the space-lattices, and there- 

 fore the ^^olumes and edge-dimensions of their 

 cells relatively expressed by the molecular 

 volumes and topic axes and confirmed by absolute 

 measurement by X-rays, are strictly comparable, 

 whereas the cells of morphotropic substances are 

 not at all necessarily comparable, and, in general, 

 are probably not so. 



Some of the drawings could well be improved, 

 but the photographic figures of biological objects 

 and X-radiograms are excellent. Before another 

 edition is printed numerous slight errors in 

 English and spelling should be eliminated with the 

 aid of some English friend. The author's power 

 of expressing himself in our language is, how- 

 ever, surprising, and often truly excellent. 



The book is an inspiring one, and well worthy 

 of the attention of both chemists and biologists. 

 If stereochemists had all been forced to take an 

 elementary course of crystallography they would 

 have avoided much of the acrimony of the long- 

 drawn-out discussion over asymmetric atoms, the 

 exact conditions being plain ; and they would 

 have recognised right away that the presence or 

 absence of a plane of symmetry is not the test for 

 optically active enantiomorphism. The reason 

 why Prof. Pope's work in this domain is so well 

 grounded, and why we listen also to Prof. Jaeger 

 with such confidence, is that both are not only 

 stereochemists, but also practical crystallo- 

 graphers. The moral is clear. 



A. E. H. TuTTON. 



ELECTRICITY METERS. 

 Electricity Meters: their Construction and Man- 

 agement. A Practical Manual for Central 

 Station Engineers, Distribution Engineers, and 

 Students. By C. H. W. Gerhardi. Second edi- 

 tion, revised and enlarged. Pp. XX + 504. (Lon- 

 don: Benn Bros., Ltd., 1917.) Price 155. net. 

 np*HE function of an electricity meter is to make 

 ^ -»■ an accurate record of the energy expended 

 in a consumer's appliances. The principles on 

 which it works are well known, but without hav- 

 ing recourse to the integral calculus it is difficult 

 to explain how it fulfils its functions. On p. 3 of 

 this book, for instance, the author has to explain 

 that what it measures is the integfal of the pro- 

 duct of two functions of the time, namely, the 

 pressure and the current. In fact, an electricity 

 meter is a most wonderful calculating machine, 

 which has been gradually evolved by the joint 

 labours of mechanical inventors, electricians, and 

 mathematicians. Although it is generallv placed 

 NO. 2536, VOL. lOl] 



in some out-of-the-way position, where it is 

 looked on with disfavour by the domestics, yet 

 it performs its wonderful operations with the 

 highest accuracy from year to year. Manufac- 

 turers also turn out many thousands of " tariff " 

 meters — that is, meters which will let you have a 

 cheaper supply at one period of the day when the 

 demand is small, and a dearer supply at the times 

 of the "peak" load. In addition, many thousands 

 of penny, sixpenny, and shilling " in the slot " 

 meters are in daily operation. In nearly all cases 

 when the electricity paid, for is exhausted, the 

 lights go out, but in some cases an intolerable 

 blinking of the lights occur. As more than one 

 coin can usually be inserted in a meter, there is 

 a dial showing the number of "unconsumed coins " 

 remaining in it. 



One good effect of the lighting restrictions will 

 be that it will make many householders read their 

 meters once a week. When they have mastered 

 this accomplishment it will begin to dawn on them 

 what an admirable servant an electricity meter is. 

 If they are skilled observers they may be able to 

 detect slight differences in its rate, but unless they 

 are very skilled, or the meter does something out- 

 rageous, it will not be safe to dispute the reading 

 with the supply company. From a long experi- 

 ence of "disputed" meters the writer finds, from 

 his official tests, that only about 10 per cent, of 

 them are outside the limits of permissible error. 

 He has known, however, a meter where the thou- 

 sand dial of the cyclometer counting train made 

 a jump of 2000 units instead of 1000, and yet on 

 inspection of the train and on an official test over 

 the disputed part of the dial it behaved normally. 

 The consumer, however, being a careful man, 

 entered the weekly readings in a book, and thus 

 proved conclusively its nefarious deed. A flaw in 

 the mechanism, discovered later, proved how it 

 happened. 



The most difficult errors to detect in meters are 

 in connection with the wheel train. Sometimes it 

 requires more power to turn it in certain positions 

 of the dials than in others, and tests to detect 

 this fault are difficult to devise. In the study of 

 friction losses great progress has been made. 

 These losses are due to the commutator brushes 

 (which are sometimes made of gold), the bearing, 

 the gearing, and air friction. The friction of the 

 bearing and gearing is practically constant at 

 all loads. The brush and air friction increase with 

 the load, and, therefore, with the speed of the 

 spindle, but the latter does not obey the parabolic 

 law. In some cases the total friction torque 

 appears to attain a minimum value at low speeds. 

 When the spindles are supported by cupped 

 diamond bearings the bearing friction is very small.. 

 Sometimes it is eliminated altogether by keeping . 

 magnetically the rotating part floating in the air. J 



This book should prove most useful for those 1 

 for whom it is written. The practical hints given "■ 

 in the chapters on "Choice of a Type of Meter"" 

 and on "Meter Testing " are good. The diagrams 

 in the latter chapter are worthy of commendation. 

 Several of the meter illustrations, however, are 

 very poor, and could have been omitted with ad- 



