564 



NA TURE 



{April 17, 1890 



condenser of known capacity with one or more Clark's 

 cells, of which the E.M.F. at any ordinary temperature 

 is now well known, and discharge the condenser through 

 the ballistic galvanometer ; or, if a sufficiently delicate 

 ampere-meter be available, the ballistic galvanometer 

 may be very accurately calibrated for steady currents, and 

 then its constant for a sudden discharge is at once known 

 by simply measuring, in addition, the periodic time of 

 vibration of the needle and its logarithmic decrement. 



The book concludes with an appendix giving the de- 

 cisions arrived at in 1886 by the Electrical Standards 

 Committee of the British Association, and the further 

 resolutions which were passed at the meeting of the 

 Electrical Congress in Paris last year, and subsequently 

 agreed to by the British Association Committee. Then 

 follow twelve sets of useful tables. 



Although we have made a few suggestions that the 

 author may perhaps like to adopt in publishing the third 

 edition of his " Absolute Measurements in Electricity 

 and Magnetism," we desire to emphasize our warm 

 appreciation of this the second edition. On every page 

 may be seen evidences of the firm grip of the subject so 

 characteristic of the author's teacher— the teacher, in fact, 

 of us all — Sir William Thomson ; and did we know of 

 higher praise than this we would give it. 



" The Theory and Practice of Absolute Measurements 

 in Electricity and Magnetism, Vol. I.," also by Prof. 

 A. Gray, is a mathematical expansion of the elec- 

 trical portion of his book on "Absolute Measure- 

 ments, &c.," the mathematical treatment of the mag- 

 netic portion being reserved for Vol. II. of the larger 

 work. As many of the remarks that we have 

 already made regarding the smaller work apply equally 

 well to the larger, it is unnecessary to criticize the 

 larger book at any considerable length. The two 

 books may be read quite independently of one another, 

 since much of the descriptive matter is the same in both. 

 If there be a fault in the larger work, we think that it 

 arises from the author forgetting that a book intended 

 initially for the University student can also be made 

 of great value to the more practical electrician if first 

 the subject-matter be arranged in propositions, or with 

 distinct headings to the paragraphs, so that it is easy 

 to find the proof of any particular fact'; and, secondly, 

 if complete proofs be given of important practical 

 problems, instead of simply deducing them as special 

 cases of more general problems. For example, a prac- 

 tical electrician may desire to see how the logarithmic 

 formula for the capacity of a cable is arrived at. Now, 

 there is no difficulty in giving a fairly short complete 

 proof of this ; but, on turning to Prof. Gray's " Theory 

 and Practice, &c.," the electrician finds that he must first 

 master the theory of charged ellipsoids ; he sees several 

 double integrals and several lines of long mathematical 

 formula in small print, and he probably decides that he 

 had better pass by that subject for the present. We hold 

 that, since the pure science of electricity owes so much 

 to its practical development, it is but fair that the pure 

 mathematician should endeavour to repay this debt by 

 stating his results and methods of proof in such a form 

 that they can be most easily grasped by anyone who 

 desires to use them, and not merely to get up the subject 

 for examination purposes. The general mathematical 

 investigations are also, of course, of great value, and we 

 are therefore glad to see in this book a fairly complete 

 mathematical treatment of Green's theorem, inverse 

 problems, electric images, problems of steady flow in 

 non-linear conductor, and variable linear flow, with its 

 application to the speed of signalling in submarine 

 conductors. 



Very interesting information is given regarding the 

 strength and torsional rigidity of the fine silk fibres used 

 in suspending galvanometer needles, followed by the 



mathematical theory of oscillations, the description of 

 the practical methods of measuring periodic times of 

 oscillation and moments of inertia, and concluding with a 

 comparison of unifilar and bifilar suspensions. The suc- 

 ceeding chapters on electrometers, the general measure- 

 ment of resistance, the calibration of the wire of a metre 

 bridge, the measurement of very low resistances, the 

 measurement of very high resistances, the determination 

 of specific resistance, contain what is given on these 

 subjects in the smaller book amplified. 



The last chapter. No. VIII., in this larger treatise, on 

 capacity, is very complete. It gives a description of the 

 most important investigations that have been made on the 

 specific inductive capacity of solids, liquids, and gases, 

 together with the mathematical theory of each experiment. 



Although we cannot but feel that the smaller of the two 

 books published by Prof. A. Gray is the more unique, the 

 larger is a very creditable production, and will be valu- 

 able as a book of reference for those who desire to con- 

 sult a shorter book on mathematical electricity than that 

 of Messrs. Mascart and Joubert. 



We now come now to the fourth book, " Electricity in 

 Modern Life," by Mr. de Tunzelmann, which is written 

 on an excellent basis, and contains a great deal of useful 

 popular information, but it unfortunately also contains 

 many unnecessary errors. For example, the statement 

 on p. II, that " a single cell of this kind," potash bichro- 

 mate, " holding about a quart of solution, is capable of 

 maintaining the light of a small incandescent lamp for 

 some three or four hours," would rather disappoint a 

 purchaser of a quart, or any size, bichromate cell, as he 

 would find it most difficult to purchase an incandescent 

 lamp that would glow with so small a difference of poten- 

 tial as one cell could produce. Again, to say in Chapter 

 II., on "What we Know about Magnetism," "Weber's 

 theory of magnetism may now be considered as raised 

 from the rank of an hypothesis to that of an established 

 fact," gives a totally wrong idea as regards our knowledge, 

 or, rather, as regards our ignorance, of the mechanism 

 of magnetism. " The face of the magnet that before 

 pointed to the north,' &c., is not exactly wrong ; but can 

 a face point towards anything? "If a current goes 

 round the solenoid in the direction of the hands of a 

 watch with its face directed towards the end from which 

 the current flows, the end of the steel bar within the end 

 of the solenoid at which the current leaves will be found 

 to be a north pole and the other end a south pole," would 

 lead the reader to imagine that the polarity of the core 

 of an electromagnet depended partly on the direction in 

 which the current flows parallel to the core, instead of 

 depending, as is the fact, wholly on the way it flows 

 round the core. 



Chapter IV., on " Force, Work, and Power," is good, 

 and the careful distinction drawn between yvork and 

 power is forcible and apt. But why does the author 

 limit the definition of a horse-power, 33,000 pounds 

 raised i foot per minute, to the " indicated horse-power " 

 of a steam-engine. 



Chapter V. deals with the " Sources of Electricity." 

 In describing the chemical action of a galvanic cell 

 formed " of a plate of zinc and a plate of copper partly 

 immersed in sulphuric acid," it is an obvious mistake to 

 speak of the action as a simple liberation of hydrogen at 

 the copper plate, and oxygen at the zinc, ard to omit all 

 reference to the formation of zinc sulphate. The first 

 part of the following statement has been experimentally 

 disproved some fifteen years ago : — " If either the copper 

 or zinc is immersed alone in dilute sulphuric acid, a differ- 

 ence of potential will be produced between the metal and 

 the liquid ; but if the two metals are immersed side by 

 side into the liquid, then no electrification can be de- 

 tected." A galvanic battery is defined by the author as 

 " a series of galvanic cells so arranged that the zinc of 



