June i, 1893] 



NA TURE 



99 



the appreciation the author may have of English work. 

 That it would have been better written if his reading 

 had been wider or his acquaintance with the English 

 literature of the subject more thorough, Dr. Waldo would 

 himself admit. The object of the book is distinctly to 

 make known in English reading circles what has been 

 effected on the Continent by the studies of Von Bezold, 

 Siemens and others, and this object is well executed. 

 The exhibition of the views of these masters in lucid 

 terms, and with a few exceptions the author makes his 

 meaning very clear, is more than a sufficient reason for 

 the appearance of the book, which will be welcomed by 

 many students, who are thus put in easy possession of 

 much abstruse work, which possibly embodies the more 

 or less crudely shaped views that they themselves have 

 held, but have been unable adequately to express. 



William E. Plummer. 



THE TRANSMISSION OF TELEPHONE 

 CURRENTS. 

 Telephone Lines and their Properties. By William J. 

 Hopkins. (London : Longmans, Green and Co.) 



IN this book the author has attempted the difficult 

 task of instructing both the student and the practical 

 man, and the result is, on the whole, more successful 

 than is usually the case. The first half of the book is a 

 text-book of the modern practice of telephone lines in 

 America, and contains a large amount of good and inter- 

 esting information on overhead and underground lines, 

 poles, insulators, wires, conduits, cables, exchanges, and 

 switchboards. This covers too wide a field to be useful 

 to a telephone engineer, as each subject is necessarily 

 treated in a cursory manner, but to an English reader it 

 is very interesting, if he knows enough of his own practice 

 to note and appreciate the points of difference. Some 

 of these indeed will make the general public thankful for 

 the restrictions under which telephone men labour over 

 here, and one of the illustrations — a street telephone 

 pole, about loo feet high, with eighteen heavy wooden 

 •cross-arms — is a testimonial to the patience of American 

 people. One or two of the explanations of facts outside 

 strictly electrical information require revision ; for in- 

 stance, the coating formed on copper wire exposed to 

 damp air is the hydrated carbonate of copper, and not 

 the chloride, as stated by the author. Also the dictum 

 on cable-i for underground circuits is somewhat curious, 

 indiarubber insulation being condemned as not impervious 

 to moisture, and liable to soften by heating, thereby 

 allowing the wire to sink through the insulation. Con- 

 sidering that in another part of the book a current of ten 

 milliamperes is given as a maximum for telephone work, 

 it is difficult to see how any appreciable heating is to 

 take place, as the current density is such as would rejoice 

 the heart of Mr. Heaphy. Again, the accusation that 

 indiarubber will not exclude moisture for a longer time 

 than anything else, makes one wonder what they make 

 it of in America. But after all these may be differences 

 of opinion, and in general the information is unusually 

 accurate, and is very clearly expressed. The only excep- 

 tion to this is in the chapter on switchboards, where the 

 frequent use of technical terms is likely to give some 

 difficulty to a student who is ignorant of practice. The 

 NO 1231, VOL. 48] 



addition of inverted commas to a technical expression is 

 small assistance, where no explanation of its meaning is 

 volunteered. 



The second half of the book rises above the text-book 

 standard, and gives a very good account of recent work 

 on the bearing of the capacity and inductance of a circuit 

 on its transmitting power, and the effect of the configura- 

 tion of itself and neighbouring wires, on its freedom from 

 cross-talk and external influences. The paper of Mr. 

 J. J. Carty, of New York, on the effects of electrostatic 

 induction from neighbouring wires is largely quoted from 

 and discussed, and his theory of the phenomena ener- 

 getically advocated. A large number of experiments are 

 described in a very clear manner, and the deductions 

 seem mostly incontestable. But the law connecting 

 length of wires that run parallel with each other with the 

 amount of cross-talk produced seems incorrect. It is 

 founded on only one series of experiments, and is given 

 as C=X',^// where C is the induced current and / is the 

 length of parallel line. It should surely be of the form 

 C = ^//(R-l-r) where R and r are the impedance of the re- 

 ceiving instruments, and of the whole length of line re- 

 spectively. 



The experiments very ingeniously show that by electro- 

 static induction the charged wire will induce a charge 

 in the neighbouring wires, and a series of such induced 

 charges in opposite directions make an alternating 

 current similar to the primary current. The compara- 

 tively high potential of a telephone wire and the ex- 

 tremely small current render this explanation the only possi- 

 bleone, and the absence of electro-magnetic inductioneven 

 from a much larger current is shown by simple experi- 

 ments. After this the method of shielding the line by 

 symmetrical arrangement, or by stranding or transposi- 

 tion of the wires is clearly explained, as the induced 

 current causing cross-talk is reversed in direction at 

 every turn, so that only the last section will affect the 

 receiving instruments. After considering air lines, the 

 author gives a careful investigation of the construction of 

 cables, with the effects of large capacity and the methods 

 of reducing it. Cross-talk is also considered, and 

 designs for non-inductive cables suggested. The un- 

 equal efficiency of composite lines from the two ends is 

 mentioned with unnecessary hesitation as to the reality 

 of the fact, and no definite explanation is offered. It is 

 found best to have the larger capacity at the sending 

 end, probably because there is a considerable loss of 

 electricity in transmission, and hence a smaller current 

 over the larger half of the line, whereas in the reverse 

 direction a large current has to be transmitted over a 

 great distance, only to be, to a great extent, absorbed in 

 the capacious cable. 



The chapter on external influences is written with im- 

 partiality and completeness, and contains an account of 

 some curious observations on the effect of tramways and 

 electric lighting wires. Some good advice is given on 

 the methods of avoiding disturbance, the double metallic 

 circuit being recognized as the Only completely satisfac- 

 tory way out of the difficulty. An account of one in- 

 stance of disturbance is sufficiently alarming, that a small 

 arc lamp was maintained in a telephone circuit during a 

 magnetic storm ! If this source were only more 

 regular, it might be another solution of what to do when 



