The Principles of Magnetism and Electricity An Elementary Text-book

Abstract

THE number of elementary text-books on magnetism and electricity probably exceeds that of text-books on any other subject. One would, therefore, naturally expect that anybody attempting to add to their number would do so with a due sense of responsibility, and endeavour to produce a book which might be regarded as surpassing those already in existence either in accuracy of exposition or in freshness of treatment. A careful perusal of the book before us has forced us to the conclusion that the author is destitute of all sense of responsibility, and not afraid to scatter error broadcast with a light heart. Seldom has it been our lot to come across an elementary text-book so full of glaring errors so boldly stated. On p. 18 the author describes a vibrational method of comparing the moments of two magnets in which the moments of inertia of the magnets are not even referred to! On p. 15 we have the startling assertion that in the case of diamagnetic bodies “the induced magnetisation is at right angles to the field” (the italics are the author's!). Could there be a greater confusion of ideas than that exhibited by the following sentence? (p. 151): “A pole of strength m will have 4πm lines of force proceeding from it, so that, if a transverse narrow cut be made across a magnet which has σ lines per sq. cm. in any normal cross-section, the field in the narrow slit H will be equal to 4πσ.” The author measures magnetic force in dynes, and difference of potential in ergs. On p. 162, in connection with the induction coil, we read: “Trowbridge has recently obtained sparks nearly seven feet in length, obtaining an E.M.F. of 3,000,000 volts, the primary current being supplied from a battery of 10,000 storage cells” (the italics are ours). Is the author serious, or does he intend playing a practical joke on his reader, by suggesting that any sane person would use 10,000 storage cells for supplying the primary of an induction coil? Had he taken the trouble to refer to Prof. Trowbridge's papers, the author would have found that the arrangement used for obtaining the 3 × 10⁶ volts had nothing whatever to do with an induction coil. On p. 163 we have the sentence: “The total value of the magnetic force within a circuit is known as the magnetic flux through the circuit.” Now, what does the author mean by “the total value of the magnetic force within a circuit”? When touching on technical matters, the author does not scruple to make various erroneous statements with an airy assumption of superior knowledge. “Theoretically,” we are told on p. 166, “every dynamo could be used as a motor and every motor as a dynamo. In practice, however, this power of reversibility is not used.” Again, on p. 170, we read: “Owing to the self-induction of each section of the armature, a certain amount of energy is used twice in each revolution to establish the current in it. This energy is lost so far as the external circuit or the effective output of the machine is concerned” (the italics are ours). This sentence shows that the author has never attempted to study the extremely complicated problem of commutation; it would, therefore, have been wiser to say nothing about it in an elementary text-book.

The Principles of Magnetism and Electricity. An Elementary Text-book.

By P. L. Gray Pp. xvi + 235. (London: Methuen and Co., 1901.) Price 3s. 6d.