NA TURE 



60 1 



THURSDAY, APRIL 2S, 1S87 



PRACTICAL ELECTRICITY 

 Practical Ehctyicily. By W. E. Ayrton, F.R.S. (London: 

 Cassell and Co., 1887.) 



PROF. AYRTON'S book on Practical Electricity 

 fills a gap, and is well fitted for the purpose for 

 which its author has designed it. The book comprises 

 the substance of the first year's course for students of 

 electrical technology in the City and Guilds Central 

 Institution at South Kensington, with some additional 

 matter, which is chiefly in small type. The subject of 

 Current is treated first, then comes Electromotive Force 

 or Potential Difference, and afterwards Resistance. This 

 is undoubtedly the logical order, though, as Prof Ayrton 

 points out, this sequence appears complicated to the 

 minds of learners, from the fact that in the definitions of 

 the Paris Congress the volt is made to depend on the 

 ohm and the ampere. The practical units, ampere, ohm, 

 volt, &c., are used throughout, but a little more space 

 might have been given with advantage to the definitions of 

 these units. Take, for instance, the definition of an ampere, 



6. After showing by means of a most instructive and well- 

 arranged experiment (Fig. i) that a current produces 

 magnetic, chemical, and thermal effects, and further, that 

 the chemical changes are the same in two or more volta- 

 meters of the same kind. Prof Ayrton proceeds : — 

 " We shall therefore define the strc7igth of a current as 

 being directly proportional to the ajnount of chemical 

 decomposition produced in a given time ; and the current 

 that deposits o'ooillSif gramme or o'oi7253 grain of 

 silver per second on one of the plates of a silver voltameter, 

 the liquid employed being a solution of silver nitrate 

 containing from 15 to lo per cent, of the salt, we shall 

 call an ampere,' and take it as our unit current." 

 But frequently a beginner will at once wish to know 

 why these special numbers, which will seem to him un- 

 necessarily comple.v, should have been taken. Why not 

 select '001 gramme of silver rather than •00111815? will 

 be an obvious question, to which no answer is given. 

 Surely, too. four significant figures would be sufficient. 

 Besides, this is not the definition of ampere adopted by 

 the Paris Congress, and it involves the experiments of 

 someone on the electro-chemical deposition of silver. 

 The experiments given in the early part of Chapter 

 II. do not need the previous definitionof the «/«// current, 

 and from them the fact that a current exerts on a magnet 

 a definite force depending on its strength and (§ 22) on 

 the dimensions and position of its circuit is established. 

 Thence the idea of the current which exerts on a definite 

 magnet a unit force is easily reached, and from this we 

 get the anipere of the Congress definition, which is found, 

 by careful experiment, to deposit so many grammes of 

 silver per second. 



So, too, the definition of the "volt" would have been 

 clearer if the excellent illustration of difference of poten- 

 tial given in § 40, Fig. 28, had been carried a little further, 

 and it had been pointed out that, just as the water loses 

 potential energy in falling from one level to another, and 

 that loss is measured by the quantity of water multiplied 

 Vvi: . x.xxv.— No. 913 



by the difference in pressure, so the electric current loses 

 energy in passing from one point to another, and that 

 loss is measured by the quantity of electricity multiplied 

 by the difterence of electrical potential. But these, per- 

 haps, are points which can be better brought out by a 

 teacher in explaining difficulties to his class. 



The book is, we believe, the first in England which 

 accepts distinctly the resolutions of the Paris Congress 

 as a basis. Another novelty to be found in it is the use of 

 the letters P. D. (potential difference) for the old abbrevi- 

 ation E.M.F. This is a change for which, if it can be 

 satisfactorily introduced and accepted, all teachers will 

 be thankful, for it will get rid of the confusion existing 

 between the resultant electrical force at a point, and the 

 electromotive force between two points, which is not a 

 force at all. 



The plan of the book has been already indicated. 

 Starting from the definition of an ampere, the various 

 means of measuring currents are described, and full 



details are given of the methods for carrying out the 

 experiments, for recording results graphically and other- 

 wise, and for drawing conclusions from the experiments. 

 The illustrations show clearly the arrangement of appar- 

 atus in each case. In Prof Ayrton's laboratory the 

 apparatus required for any one series of observations is 

 mounted permanently on the same board ; the student 

 finds everything ready, and the necessary connexions 

 made when he begins. As to the desirability of this, 

 there will no doubt be some difference of opinion ; but 

 with large classes of beginners some such plan is necessary. 

 Thus, Fig. 15 shows the apparatus for investigating the 

 action of a current on a magnet. The large coil C c is so 

 arranged that the current can be made to traverse it 

 eight, twelve, or sixteen times, and its effect on the mag- 

 net gg observed ; or the coil c c can be replaced hy cc 

 of half the radius, which has four turns on it ; or, again, 

 cc can be used simultaneously with cc, the current being 



' We .ire indebted to Messrs. Cassell for the blocks used to illustrate this 



