602 



NA TURE 



[April 2%, 1887 



sent in opposite directions througli the two. 7f w is a 

 resistance by means of whicli the current may be varied 

 at will, and G a galvanometer. 



Chapter III. introduces us to difference of potential, 

 and here the writer describes in a practical form the 

 experiments for the verification of some of the ordinary 

 laws of statical electricity. 



Ohm's law and its proof follow in Chapter I\'., and it 

 is shown that the resistance of a conductor remains un- 

 changed so long as its other physical conditions are the 

 same. § So gives the definition of the "legal ohm." It 

 is not quite accurate to speak of its having been legalised, 

 at any rate in this country; though, as the letter from the 

 B.A. Committee on Electrical Standards, which is printed 

 at the end of the preface, shows, the question of its adop- 

 tion as a legal standard is now before the Government. 

 Various methods of comparing resistan:es follow, with 

 full practical details, but the important one due to 

 Prof G. C. Foster, for comparing two nearly equal resist- 

 ances, has been omitted from § 97. 



Batteries and other forms of current-generators are 

 next considered, with details as to their construction and 



use ; and an account of the methods of determining thei 

 E.i\I.F. and resistance is given. 



"Insulation" is the title of Chapter VI., and many o 

 the practical hints given under this head are of grea 

 value. 



In the next chapter, condensers, and the methods of 

 constructing them and of measurnrg their capacities, are 

 treated of. We find also a chapter devoted to com- 

 mercial ammeters and voltmeterb, which gives in an 

 easily accessible form particulars as to some of the best 

 known of these instruments, with practical methods of 

 testing and calibrating. In Fig, 157 we have the 

 arrangement of apparatus for comparing an ammeter 

 with a silver voltameter. R is an adjustable resistance 

 of a useful character, made of a number of washers of 

 carbonised cloth which can be pressed into close contact 

 by a screw. Latimer Clark's cell is described in § 214, in 

 the H form. A simpler pattern consists of a test-tube 

 with a platinum wire sealed through the bottom. The 

 end of this is covered with pure mercury, and above this is 

 a layer of mercurous sulphate, with a saturated solution 

 of sulphate of zinc above all. A rod of clean zinc dips 



into the zinc sulphate passing through the cork which 

 closes the test-tube, and the whole is kept tight with 

 marine glue. 



In an appendix Kirchhoff's laws of divided circuits are 

 considered. The method of treating such problems 

 introduced by Maxwell, and which has recently been 

 illustrated at some length by Prof. Fleming, is also 

 referred to. 



Another point of importance which calls for notice is 

 the number of examples introduced by way of illustration ; 

 some of these are completely worked out, others left as 

 exercises for the student. 



The book concludes with some specimens of the 

 instructions for experiments which are given to students 

 at the City and Guilds Institution, with the apparatus with 

 which each experiment is performed. Of these instruc- 

 tions there are four ; and many who use the book will 

 wish their number was larger. We will close this notice 

 by quoting the last of them. 



To calibrate an ammeter by means of a silver volta- 

 meter (the apparatus required has been figured above) : — 



" Prelimin.\ry. — The voltameter consists of a plati- 

 num dish containing a 25 'per cent, solution of silver 

 nitrate, and in which a silver plate is immersed. An 

 adjustable carbon resistance is provided, by means of 

 which the current passing through the voltameter can be 

 maintained constant during each experiment, and can be 

 varied in the different experiments. 



" EXPERIMENT.S. — (i) Carefully clean, dry, and weigh 

 the platinum dish, the approximate weight of which is 78 

 grammes. 



" (2) Pour the solution of silver nitrate into the dish, 

 and place it on the three brass pins provided for its re- 

 ception, and which are electrically connected with the 

 left-hand binding-screw on the board. Immerse the 

 silver plate in the solution, and clamp it in such a posi- 

 tion that its edges are equally distant from the sides and 

 bottom of the dish. 



" (3) Turn the small milled head at the top of the 

 ammeter so that the pointer of the ammeter comes oppo- 

 site the zero on the scale, if not there already. Place the 

 copper connecting-wire in the mercury cups marked .\ 

 and c (which cuts out the voltameter), and adjust the 

 carbon resistance until a convenient current flows round 

 the ammeter. Remove the connecting-wire. 



" (4) Quickly insert the connecting-wire in the mercury 

 cups marked A and b, carefully noting the instant at 

 which the circuit was completed. Allow the current to 

 pass for a convenient time (ten to thirty minutes, accord- 

 ing to the strength of the current used), and keep the 

 current constant by the adjustable resistance. Note the 

 temperature of the room during the experiment, and, at 

 the end of the interval decided on, quickly break the 

 circuit. 



" (5) Empty the solution from the dish into its bottle, 

 and carefully wash the deposited silver with distilled 

 water. Then fill the dish with distilled water, and 

 allow it to stand ten to fifteen minutes .Again wash with 

 water, alcohol, and ether, dry over the spirit-lamp, and 

 cool in the desiccator. 



" (6) Carefully determine the increase of weight due to 

 the silver deposited on the dish. 



" (7) Calculate the strength of current used in the 

 experiment, assuming that one ampere deposits ri 181 5 

 milligrammes of silver per second. 



" (8) Repeat the experiment with several different 

 strengths of current. 



" (9) Tabulate your results in some convenient form, 

 and write them with your name on the card, on which you 

 will find recorded the results of previous experiments." 



