June 27, 1889] 



NATURE 



201 



of telephony as a practical art, and the authors have 

 certainly succeeded in inclosing within the compass of 

 a handy octavo volume a vast amount of well-arranged 

 information on a subject hitherto unrepresented in 

 English by any systematic treatise. After two chapters, 

 comprising about twenty pages, on sound and speech, 

 and on such parts of electrical theory as are more imme- 

 diately connected with the action of the telephone, the 

 authors proceed to deal with the construction of the 

 telephone, and treat in detail the subject of transmitters 

 and receivers of all kinds. This part of the book is very 

 interesting, giving as it does an account of the principal 

 forms of telephone receivers and transmitters which were 

 the outcome of the marvellous activity of telephonic 

 research aroused by the publication of the inventions of 

 Bell, Edison, and Hughes. In chapter x. come tele- 

 phone lines and cables, and modes of installing them ; 

 then chapters on auxiliary apparatus, and on terminal 

 and intermediate stations, lead up to the important subject 

 of telephone exchanges, and appliances connected with 

 their working, to which chapters xiv. to xx. are de- 

 voted. Long-distance telephony is introduced in chapter 

 xxi., and systems of translation between the terminal 

 networks, and the return wire or other induction avoiding 

 circuit between the two places, are fully described. Various 

 problems of practical telephony are then discussed, such 

 as multiplex telephony, and the numerous devices for 

 enabling several subscribers to work in one circuit. 

 Chapters on the telephone as applied to the telegraph 

 service, its military uses in camp and in the field, and 

 finally some miscellaneous although important scientific 

 applications of the instrument, conclude the work. 



Of the main body of the work the contents of which 

 are briefly analyzed above, we have no remark to make 

 that is not commendatory. As has been already indi- 

 cated, the work is full of most valuable practical details 

 of actual working systems of telephony. The descrip- 

 tions of complicated apparatus and appliances are full 

 and clear, and bear everywhere the stamp of the work 

 of men accustomed to exposition, and professionally 

 acquainted with the subjects of which they treat. 



In our examination of the work, we have noted a few 

 points in which perhaps improvement might be effected 

 in a second edition. Most of these occur in the pre- 

 liminary chapters on the theory of the telephone, a part 

 of the subject in some ways much more difficult to treat 

 successfully than that which follows. For example, such 

 . a phrase as that on p. 14 — that in a current following the 

 simple harmonic law of variation with the time the 

 electricity flowing is, " so to speak, thrown into undula- 

 tory motion" — isveryaptto create an erroneous impression 

 on the mind of a learner, and in no way describes what 

 actually takes place. What is thrown into undulatory 

 motion is not the electricity conveyed but the medium 

 which forms its vehicle. 



On p. 16 the phenomena of induction of currents are 

 hardly " described with due precision." It is stated that 

 "if there be a magnetic field, and a conductor in that 

 field, any change in that field will produce the conditions 

 that determine a current in that conductor." No doubt 

 it is stated innnediately afterwards that, " if a conductor 

 forming part of a closed circuit be moved across a mag- 

 netic field in a direction at right angles to the lines of 



force in that field, a current will be induced whose 

 strength is proportional to the strength of the field and to 

 the rate at which the conductor cuts across the lines of 

 force," but it nowhere appears that a change of the mag- 

 netic induction through the circuit is the one determining 

 condition of an induced current. The authors, indeed, 

 as if to sum the matter up, conclude this passage with the 

 unqualified statement, which, as it stands, is not true ex- 

 cept under certain conditions which are not stated : " In 

 fact, currents are produced in a closed circuit placed in a 

 magnetic field, whenever any change whatever occurs in 

 the intensity of that field." If the circuit be so placed in 

 the field that there is zero magnetic induction through it, 

 the field may be absolutely annulled without producing 

 any current whatever. 



Again, on p. 18 it is stated that "the energy of the 

 current in a coil at any moment is expressed by the pro- 

 duct of the electromotive force (E) at the terminals of the 

 coil producing the current, and the current itself (C), that 

 is, W = E C." Now, what is here called the "energy of 

 the current " is in reality the activity or time-rate of 

 working of the current. The energy of the current at any 

 instant is the energy which would be evolved in the form 

 of a spark, or otherwise, if the current in the circuit were 

 at that instant annulled. It is just this kind of misuse of 

 the word energy that has led to the popular confusion (very 

 common among the so-called " practical " (!) men who 

 have applauded the Quixotic crusade against theory and 

 " theoreticians ") between work and rate of working, and 

 to the astounding proposition, not yet exploded in all 

 quarters, that the efficiency of a motor is a maximum 

 when half the whole energy spent is wasted. 



There are various other points and some omissions in 

 the introductory and theoretical portions which we had 

 noted. The remarks on self-induction might be improved, 

 and it would have been well to point out here that 

 conductors carrying rapidly-alternating currents (and 

 telephone wires certainly do this) have, as Lord Rayleigh 

 has shown, a virtually increased resistance due to the 

 concentration of the current in the outer part of the wire. 

 Another point is the apparently unguarded application, 

 made at pp. 123, 124, of the results of the theory of a 

 slowly worked submarine cable to the transmission of the 

 rapidly alternating currents of telephones. The state of 

 the case is much less simple than the authors here make 

 it appear. Also, it is not possible, with any approach to 

 accuracy, to regard copper wires, in this connection, as 

 " virtually free from electro-magnetic inertia." But we 

 have said enough as to these blemishes. They can be 

 removed by careful extension and rewriting of the intro- 

 duction. After all, it ought to be recognized that it is 

 impossible to give in a book on a branch of electro- 

 technics any statement of theory which can supersede 

 that full and detailed treatment which is indispensable, 

 and which must be sought in systematic treatises on 

 electrodynamics. 



As to the more purely technical portion of the work, 

 we have only to repeat that it is full and trustworthy, and, 

 moreover, remarkably well illustrated. There are several 

 statements made by the authors which might be ques- 

 tioned, but as these are in many cases matters of opinion 

 rather than of actual fact, we need not enter into them. 



In conclusion, we have to say that this book is a 



