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    ENGINEERING data of the manufacture and design of electrical machinery and also descriptions of tests which are useful in laboratories and testing rooms are given in this work. The book opens by discussing the electric strength of insulating materials, and we gather that electric strength has something to do with “average breakdown voltage.” The impression left on the reader, however, is that possibly there is no such thing as “electric strength” after all. The scientific man will be appalled at the many trade names of insulating materials. The methods of testing devised by members of the various research committees have strictly practical ends in view, and a large “tolerance” has to be allowed for the results. We have a difficulty in understanding what is meant by saying that the power factor for clear micas is 0.003. The phrase “dielectric constant” is rather an unfortunate one, when we have to discuss how it varies with temperature, humidity, etc. In the appendix it is called specific inductive capacity. The type in which the mathematical tables is printed is difficult to read. In our opinion it is time that the so-called “international and B.O.T. units” were placed on the scrap-heap. The ampere, the volt, and the ohm have been determined with an accuracy far in excess of that required in commerce.

    Manufacture, Design, and Laboratory Work.

    Compiled and collated by D. V. Onslow. (Electrical Engineers' Data Books, Vol. 2.) Pp. xvi + 276 + cxcv + 9. (London: Ernest Benn, Ltd.; Radio Press, Ltd., 1925.) 15s. net.

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