April 1 7, 1 890] 



NATURE 



565 



each cell is connected with the copper of the next cell." 

 What, then, is a collection of galvanic cells arranged in 

 parallel, in which the zinc of every cell is connected with 

 the zinc and not with the copper of the next ? Excluding 

 these mistakes, this chapter is fairly good ; the matter, 

 however, is rather too condensed to be intelligible to a 

 reader not previously acquainted with the subject. 



Chapter VI. deals with '' Magnetic Fields," and in order 

 to lead up to the mapping out of a magnetic field, the 

 mapping out of the gravitation field of force in which a 

 comet moves is first explained. But it appears to us 

 that, since the magnetic field can be easily mapped out 

 with iron filings in the well-known way, while the con- 

 ception of a gravitation field of force is a less simple 

 matter to grasp, Mr. de Tunzelmann has in this case 

 explained the easy by means of the difficult. 



The next chapter, on " Electrical Measurement,'' is 

 quite correct, but, in view of the great difficulty that is 

 always experienced by a beginner in grasping the idea of 

 measuring so intangible a thing as electricity, would not 

 this subject have been made clearer if not merely the 

 scientific definitions of the electrical units had been given, 

 but in addition an illustrated description of the meters 

 used to measure amperes, volts, &c. .>* 



Chapter VII., on " Magneto and Dynamo Electric 

 Machines,'' gives a short comprehensive description of 

 the principles of these machines, but, in order that the 

 reader might understand what a real dynamo was like, 

 we think it would have been better if the author had 

 given in this chapter at least some one of the illustrations 

 representing real dynamos which appear in other parts 

 of this book. The symbolical figures that are given are, 

 as the author mentions, taken from Dr. Thompson's 

 book on dynamo machinery, and are very clear, with one 

 exception, that while in each case the direction of the 

 current in the wires attached to the brushes is indicated 

 by arrows, the direction in which the wire is coiled on the 

 armature is omitted, hence such statements as " the 

 arrows show the current in the circuit when the armature 

 revolves as indicated by the position of the brushes," are 

 just as likely to be wrong as right, and tell the reader 

 nothing. When comparing the series dynamo with the 

 shunt dynamo, the author says that the former " will not 

 begin to excite itself until a certain speed has been 

 obtained depending on the resistance of the circuit." 

 From this the reader might easily be misled into thinking 

 that the shunt machine did not possess a similar defect. 

 Further, he states, as " the principal objection to shunt- 

 wound machines," that the self-induction of the field- 

 magnet coils leads to the result that " any variation in 

 the speed produces its effect upon the lamps before the 

 current in the existing circuit has had time to undergo a 

 sensible change." But, as a matter of fact, the self- 

 induction of the field-magnet coils of a shunt machine is 

 an advantage^ not a disad^ia/itage j for suppose that the 

 speed increases, then the E.M.F. increases, this causes 

 the difference of potentials between the lamp-mains to 

 increase, which not only sends a larger current through 

 the lamps, but also through the shunt coils. This 

 strengthening of the field causes an additional rise in the 

 E.M.F. of the machine, and therefore in the terminal 

 difference of potentials. Consequently the second ob- 

 jectionable rise is hindered, and not accelerated, by the 

 self-induction of the shunt coils ; hence self-induction of 

 the field-magnet coils of a shunt machine makes the 

 difference of potentials between the lamp-mains less 

 quickly, and not more quickly, affected by a change in 

 the speed of driving. In speaking of alternate-current 

 dynamos, it is stated that " in some machines the arma- 

 ture remains at rest, and the field-magnets are made 

 to rotate ; and in this case no slidmg contact is required, 

 the terminals of the main circuit being attached per- 

 manently to the armature." But the statement is mis- 

 leading, since at least one sliding contact must always 



be used ; only when the armature is fixed it is to lead the 

 exciting current into and out of the rotating field-mag- 

 nets that one, and in some cases two sliding contacts are 

 employed. 



Chapters IX., X., and XI., on " The Story of the Tele- 

 graph," " Overhind Telegraphs," and on " Submarine 

 Telegraphs," are excellent, we may almost say exciting, 

 and they lead the reader on like the pages of a well- 

 written novel. It is not right, however, on p. 112 to 

 say, when speaking of telegraphing with sounders, *' The 

 dots are formed by giving a sharp stroke to the key ; the 

 dashes by depressing it more slowly," since a dash is 

 formed not by depressing the key slowly, but by holding 

 it down for a time when depressed. Whether a key be 

 depressed slowly or quickly makes no difference in the 

 signal received ; what the receiver listens for is the in- 

 terval between the commencement of the current pro- 

 duced when the key is fully depressed and its termination 

 when the key is caused to begin to rise again. We pre- 

 sume that when the author says, on p. 129, '' The cups " 

 of insulators " are made of such a form as to expose the 

 upper portions freely to the cleansing action of the rain 

 while the lower portions are shielded from the rain so as 

 to keep them fairly dry," he means by " upper portions " 

 the outside of the cup of the insulator, and by the " lower 

 portions" the inside ; but if so, he has a curious way of 

 expressing himself. The " speaking galvanometer " used 

 in receiving the message sent through a submarine cable 

 is not, as the author describes it on p. 150, an astatic 

 galvanometer ; and even if two magnets were employed so 

 as to form an astatic combination, it would be quite 

 wrong to say " each of them is attached to the back of 

 a small mirror," since, unnecessary as it would be to use 

 two suspended magnets in a speaking galvanometer, it 

 would be still more useless to employ two suspended 

 mirrors. But these are not very serious errors in chapters 

 that are so good. 



Chapters XII. and XIIL, on "The Telephone " and 

 "The Telephone Exchange System," appear to us to be too 

 much of the newspaper special correspondent order, the de- 

 scriptions in several cases being very meagre, suggestive 

 rather than descriptive, in consequence of the author 

 having attempted to touch on too many different things. 

 For instance, if the photophone had to be described at 

 all, it required more than one page and a quarter, in- 

 clusive of the illustration, to make it intelligible ; in fact, 

 unless the framework of the telephones and the gentle- 

 man's head which is betwien them in Fig. 53 are all 

 composed of electrically conducting material, we fail to 

 see how the instrument, as there depicted, works at all. 

 Some very interesting information is given on the subject 

 of telephone exchanges, and we should have liked to have 

 had much more information on this electrical subject ; for 

 example, greater details regarding the switches, the reasons 

 of the babble of many conversations that everyone hears 

 who tries to use the telephone in London, &c. ; space, if 

 necessary, being economized by the omission of the de- 

 scription of the non-electrical instruments, the grapho- 

 phone and phonograph. 



Chapter XIV., on the " Distribution and Storage of 

 Electrical Energy," is very good and forcible. We fail, 

 however, to see how the use of the three-wire system 

 leads to the result stated on p. 199, that " a variation 

 of 5 percent, in the E.M.F. in the mains would produce 

 a variation of only 2^ per cent, at the lamp terminals." 



The next chapter, XV., on " Electric Lighting," is also 

 very good ; "flashing" the filament of an incandescent 

 lamp, however, does not mean sending a current through 

 the filament while the lamp is attached to the Sprengel 

 pump, but sending a current through the filament and 

 making the filament incandescent when in a hydro- 

 carbon atmosphere before it is placed inside the glass 

 bulb of the lamp. Is it a fact that "the Shaftesbury 

 theatre " is " now lighted by incandescent electric lamps ? " 



