March i6, 1893] 



NATURE 



461 



Chapters II, and III. are devoted to measurements of 

 magnetic quality by the magnetometer and ballistic 

 methods. With respect to the former very full informa- 

 tion is given as to the construction of the apparatus and 

 its use. 

 The earth's coil as a means for calibrating the 

 Ballistic Galvanometer is fully explained, as also 

 that of a solenoid and current. Mention is not made 

 of a convenient method of calibration in which 

 the quantity of electricity passed is given directly 



A T 



by Q = ; where A is the deflection corrected for 



decrement ; a is the steady deflection due to unit current, 

 and T is the periodic time of the ballistic needle. Here 

 a and T are quantities very readily obtained. 



The chapter concludes with a full description of Dr. 

 Hopkinson's " Bar and Yoke " method. 



Chapter IV. contains valuable information with regard 

 to curves of induction and hysteresis in the case of 

 wrought iron, steel, and cast iron, which will be of use 

 to the electrical engineer in the design of dynamo electric 

 machinery. The effects of annealing and stretching iron 

 are brought forward and well illustrated. 



The next chapter, on magnetic hysteresis, is perhaps 

 the most important in the book. It commences by giving 

 a clear definition of hysteresis, the effects of which are 

 amply illustrated by curves, and stress is laid upon the 

 definition of permeability as being the ratio of B to 

 H with certain limitations. 



The dissipation of energy through magnetic hysteresis • 

 — which plays such an important part in the design of 

 cores for transformers, and the armatures of dynamos — is 

 fully treated. 



The remarks on magnetic viscosity towards the end 

 of the chapter are worthy of very careful consideration. 

 The author points out that in the case of quick cycles, 



/ Hrtfl may be widely different from what is found to be the 



case by static methods, and further remarks that experi- 

 mental evidence is wanting under this head.' 



Chapter VI. treats of magnetism in weak fields. The 

 author refers to experiments by Lord Rayleigh and 

 himself, in which the time effect upon magnetism is 

 clearly shown— the creeping up of the magnetism going 

 on for a considerable time. 



Magnetism in strong fields is discussed in Chapter 

 VII. The " Isthmus Method " introduced by the author 

 and Mr. W. Low in 1887 is capable of producing mag- 

 netic fields of enormous strength. In giving his 

 conclusions from experiments by the isthmus method 

 the author states, " there is apparently no limit to the 

 value to which the induction may be raised. But, when 

 we measure magnetisation by the intensity of magnetism 

 I, we are confronted with a definite limit — a true satura- 

 tion value, which is reached or closely approached by 

 the application of a comparatively moderate magnetic 

 force." 



A full account of Dr. Hopkinson's researches on the 

 effect of temperature on magnetism is given in Chapter 

 VIII., and reference is made to the identification of 

 recalescence with recovery of the magnetic state. 



1 For recent experiments upon Magnetic Viscosity see a paper by J. 

 Hopkinson, F.R.S., and B. Hopkinson in Electrician, September 9, i8q2. 



NO. 1220, VOL. 47] 



In the latter part of the chapter hysteresis, in the 

 relation of magnetic susceptibility to temperature, 

 is dealt with ; and mention is made of the wide 

 range of temperature through which the alloys of 

 iron and nickel may exist in either the magnetic or non- 

 magnetic state. 



Reference is made to the researches on recalescence of 

 Osmond, who has since shown the marked influence of the 

 initial temperature, and the rate of cooling on recal- 

 escence in the case of chromium steel. Dr. Bottomley 

 has shown that the alloys of chromium and steel in the 

 unannealed state have exceptionally high magnetic 

 qualities, which are confirmed by experiments of Dr. 

 Hopkinson. 



In Chapter X. the magnetic circuit is discussed, and 

 the way in which it is applied to the design of dynamo 

 electric machines and transformers. Reference is made 

 to the important work of Drs. J. and E. Hopkinson and 

 Kapp upon this subject — more especially in con- 

 nection with dynamo electric machinery. In pursuing 

 the analogy of the magnetic circuit to the ordinary 

 conduction equation. Prof. Ewing lays stress upon 

 the fact that the permeability (/x) is a function of the 

 induction (B), and this is a point which cannot be too 

 strongly urged. Much that is in this chapter has great 

 practical importance — the treatment of the subject being 

 considered from a graphical, as well as analytical, point of 

 view. The chapter ends with an account of the influence 

 upon magnetism by cuttings and the compression of 

 joints in magnetic circuits. 



The last chapter gives a complete account of the 

 different theories of magnetism. Weber's theory is dis- 

 cussed with modifications by Maxwell and Wiedemann, 

 to which are added Prof. Ewing's own views of the 

 subject. He goes on to show that the reduction of 

 hysteresis by vibration is explained by the molecular 

 theory of magnetism, — and further supposes that time- 

 lag in magnetism can be accounted for by it. The book 

 ends with an account of Ampere's hypothesis of 

 magnetic molecules. E. Wilson. 



OUR BOOK SHELF. 



Forschungsberichte aus der Biologischen Station zu 

 Plon. Theil I. Faunistische und biologische Beobach- 

 tungen am Gr. Pldner See. Von Dr. Otto Zacharias, 

 Direktor der Biologischen Station. (Berlin : R. Fried- 

 lander und Sohn, 1893.) 

 The first report of investigations from the biological 

 station of Plon, in Holstein, has just been issued. It 

 is a journal of 52 pages with one plate, bearing on the 

 front of the cover a neat representation of the turreted 

 three-storey building reflected in the quiet waters of the 

 inland lake, and on the back a list of the regulations 

 observed in the management of the station. 



In his introductory remarks the Director, who has 

 already made his views known with regard to the im- 

 portance of freshwater laboratories in the pages of several 

 German scientific periodicals, gives a brief sketch of the 

 advance already made in this direction in Italy, France, 

 and America. 



The first paper gives a list of the fauna at present 

 known to inhabit the lake. This occupies seven pages ; 

 and fourteen names, being printed in italics, signify that 

 they are new to science. The new species and genera 

 are treated in detail in the second paper. The greatest 



