REVIEWS 53S 



A short account of rectifiers or "electric valves" would be of value in the 

 section on alternating currents, which, by the way, cannot afford to ignore the 

 oscillograph and cathode-ray detectors which give experimental illustration of 

 the character and form of alternating currents under different conditions. Such 

 devices have great utility and bring the subject home to the student in vivid 

 fashion. 



A small correction may as well be inserted in the account of the Central 

 London Railways electrical equipment. The company has not for some years 

 hauled its trains by independent locomotives, as is here remarked. 



We take exception to the form of the statement (p. 652) that " pure white light 

 is in reality composed of rays of seven different colours— red, orange, yellow,, 

 green," etc. The intention is of course evident and well meant, but the statement 

 (repeated elsewhere) may find a student who would leave it with quite a wrong 

 conception of the infinite gradation of the colours in the luminous spectrum. With, 

 this very slight exception the section on lighting is admirable. 



The incandescent gas mantle invented by Auer von Welsbach suddenly reduced 

 the cost of gas lighting to one-third of what it had been before. Electric lighting 

 received a temporary set-back, but has now come into its own (again primarily at 

 Welsbach's hands) with the invention of the new metallic filament lamps, the best 

 of which boast about three times the efficiency of carbon lamps and a longer life. 

 Such lamps on low voltage circuits offer very real competition to gas. Tantalum 

 lamps we find adequately treated in the work before us, but it can fairly be ,said 

 that the recent remarkable development in wire lamps has depended chiefly on the 

 introduction of the different types of tungsten lamps. Such lamps have an efficiency 

 per candle power not far from double that of tantalum lamps ; their superiority is 

 largely due to the higher radiating properties and to the very high positive 

 temperature coefficient of resistance of tungsten. The next revision which this 

 book undergoes will entail drastic rewriting of the section on incandescent lamps. 



We were glad to find a full account of Mansbridge's tin-foiled paper method of 

 constructing condensers. Nothing, however, is said as to the construction and 

 reliability of mica condensers, nor as to the attempts which have only recently 

 resulted in producing for the first time really serviceable and efficient glass- 

 condensers, such as those of the Moscicki type, which represent probably the most 

 satisfactory and practical high-tension condenser for commercial work now on the 

 market. 



Enough has been said to illustrate the difficulties which beset the authors of a 

 book which covers much ground in a very progressive applied science. There is 

 no trouble in suggesting alterations in many sections since this book went to press. 

 But the fact must not, and cannot, be held in disparagement of the material offered 

 to us by the authors, for as to its excellence there can be no doubt. We noticed 

 but few misprints, none of them important. The style throughout is attractive, 

 and the authors write with a facile pen and a proper appreciation of a student's, 

 dilemmas. 



G. W. C. Kaye. 



An Introduction to Electricity. By Bruno Kolbe. [Pp. 430.] (Kegan Paul,. 



Trench, Triibner & Co. ioj. 6d.) 

 An Introduction to Electricity is a translation of the second German edition 

 of Prof. Kolbe's lectures delivered at St. Petersburg. It is divided into two. 

 sections, dealing respectively with static and dynamic electricity, and concludes 

 with an appendix containing Historical Remarks, Repairs, and Practical Hints. 

 The sequence followed is good, and the experiments are usually well described. 



