April I J, 1890] 



NATURE 



56J 



''Absolute Measurements in Electricity and Magnetism," 

 by Prof. A. Gray, is a most interesting book to read. 

 It opens with a detailed description of (Jauss's methods for 

 determining the horizontal intensity of the earth's mag- 

 netism, and with an account of the results of the measure- 

 ment of the variation, produced by a unit field, on the 

 magnetic moments of steel magnets of different sizes 

 tempered to different degrees of hardness. If it be 

 desired to determine the magnetic moment of a bar- 

 magnet as well as the horizontal intensity of the earth's 

 magnetism, which is of course necessary when variations 

 of the magnetic moment of a bar are in question. Gauss's 

 methods are admirable. But if the value of H is all that 

 is needed, then the simpler method of employing an 

 earth inductor with a ballistic galvanometer, which is 

 described on pp. 317-21, might well be employed. It 

 would, therefore, have been well to give a reference to 

 this method in the first two chapters, which are mainly 

 devoted to the determination of H. 



Next follows a concise statement of the various ways 

 of defining the absolute current, and a fairly complete 

 chapter on standard galvanometers. In Chapters IV, 

 and v., and in Chapter XL, to which reference is made, 

 there is given the ablest description of the dimensions of 

 the electric and magnetic units that we have ever read. 

 It is both correct and comprehensible, which is saying a 

 very great deal for an exposition of a subject which, as 

 usually explained, generally leaves even a thoughtful 

 student semi-dazed as to whether the dimensions are the 

 dimensions of the unit, or the dimensions of a quantity 

 measured in the unit. Indeed, the early reports of the 

 Electrical Standards Committee of the British Associa- 

 tion were actually wrong on the very subject of dimen- 

 sions, so that " V " was regularly defined as the ratio of 

 the electrostatic to the electromagnetic unit of quantity 

 instead of as the reciprocal of that expression. 



The volt, ohm, ampere, coulomb, watt, and joule are 

 also explained and defined in Chapter V., and Prof. Gray 

 gives Sir W. Thomson's expression "activity" for the 

 rate of doing work. He does not mention, however, that 

 the equally short word " power " is regularly employed 

 with this signification. 



Chapter VI. is devoted to the laws of the currents sent 

 by galvanic cells through single and parallel circuits, and 

 through any branch of a network like that of the Wheat- 

 stone's bridge. A neat proof is given of the arrangement 

 of a given number of cells that sends the greatest current 

 through a fixed resistance, and the reader is very properly 

 warned against confusing the arrangement which develops 

 maximum power with the most economical arrangement. 



In Chapter VII. we have a complete description of Sir 

 William Thomson's meters, but, as the book is a scientific 

 treatise (in fact, a very good scientific treatise) and not an 

 instrument-maker's catalogue, we think that the author 

 would have done himself more justice had he described, 

 in addition, some of the other many forms of electric 

 meters in common use at the present day for carrying out 

 the same measurements. Further, in view of the large 

 experience that the author of this book has probably had 

 with Sir W. Thomson's meters, it would have been well 

 had there been a description not merely of the advantages 

 of these instruments, but also of their disadvantages, a 

 subject no one would be more willing to discuss than the 

 inventor himself On pp. 133-35 is given a very simple 

 proof of the ordinary formula for the quadrant electro- 

 meter, but the reader is not here warned that the formula 

 may give an answer many per cent, wrong in practice. 

 On p. 302 it is stated that this formula may be 

 slightly wrong if the aluminium needle of the electrometer 

 be not accurately adjusted relatively to the quadrants, but 

 this, we fear, is rather misleading, since it is further stated 

 that " if the needle hangs at its proper level, and is other- 

 wise properly adjusted, and the quadrants are close, the 

 equation may be taken as accurate enough for practical 



purposes," a conclusion regarding which we understand 

 there is grave doubt. In this chapter the very importawS^ 

 subject of calibrating instruments by the use of the sii^wr" 

 or the copper voltameters is fully entered into. The Ikrge- 

 amount of valuable work done on this subject by the 

 author's brother. Prof, T, Gray, of which a description is- 

 given, endows this chapter with an authoritative character. 

 Chapter VIII. commences with the construction amd^ 

 use of the various forms of Wheatstone's bridges, th«r 

 description of the modes of using them, and hints as to 

 the care of a resistance box. The methods for calibratii^ 

 relatively and absolutely the wire of a bridge devised by 

 Matthiessen and Hockin, Foster, T. Gray, and D. M. 

 Lewis are discussed at length, and specimens given of 

 the actual results obtained at University College, North 

 Wales,by the use of these methods. The ingenious bridges, 

 which have been arranged by Sir W. Thomson, Matthies- 

 sen and Hockin, Tait and T. Gray, for measuring very low 

 resistances, are fully entered into, and the construction of 

 standard coils, the measurement of high resistances, and 

 of the resistance of a battery finish a chapter of especial 

 interest. The method of measuring the resistance of a 

 battery, proposed several years ago by Sir Henry Mance, 

 is condemned by Prof Gray as being " so troublesome as 

 to be practically useless," on account of " the variation of 

 the effective electromotive force of the cell produced by 

 alteration of the current through the cell which takes 

 place when the key is depressed." We think that it should 

 have been stated that this is not a defect especially of 

 Mance's method, but of all methods for measuring the 

 resistance of a battery based on the alteration of a steady 

 current by the alteration of the resistance in the battery 

 circuit. Would it not also here have been well to 

 describe and discuss the condenser method of measuring 

 a battery resistance, as it is the one to which the fewest 

 objections can be raised .'' 



Good as are all the chapters in this book, the next one. 

 Chapter IX., on " The Measurement of Energy in Electric 

 Circuits," is so good that it takes the palm. It com- 

 mences with the practical methods of measuring the 

 power and efficiency of motors and secondary batteries ; 

 the construction and employment of activity meters (watt- 

 meters) ; and then discusses very fully the laws of alternate 

 currents, the mathematical theory of alternate current 

 generators singly, or coupled in parallel or in series ; the 

 theory of the action of an alternate current generator 

 supplying current to an alternate current motor ; the true 

 method of measuring the power given to any circuit by 

 an alternate current ; and the error produced when an 

 ordinary watt-meter is employed. The work of Joubert, 

 Hopkinson, Potier, Ayrton and Perry, and Mordey on this 

 subject is summed up in a masterly fashion. Chapter 

 IX. is, in fact, the most complete exposition of many 

 problems connected with the all-important subject — the 

 electrical transmission of energy by alternate currents — 

 that is to be found in any existing text-book, and espe- 

 cially in a small octavo text-book, that can be easily 

 carried in one's coat pocket. 



In Chapter X. the measurement of intense magnetic 

 fields is dealt with, and a description is given of ingenious 

 methods proposed by Sir W. Thomson for measuring the 

 force on a conductor conveying a known current placed 

 in the magnetic field, and so determining the strength of 

 the field. The ordinary method of ascertaining the 

 strength of a magnetic field by suddenly withdrawing a 

 coil, of known area and number of convolutions, attached 

 to a ballistic galvanometer, is described. But in order to 

 ascertain the constant of the ballistic galvanometer, the 

 author only gives the old method of observing the swing 

 of the needle when a large coil is turned in the earth's 

 field, a method which necessarily requires for its employ- 

 ment a previous knowledge of the strength of the earth's 

 field at the place. A far simpler method of ascertaining 

 the constant of a ballistic galvanometer js to charge a 



