204 REroRT— 1895. 



henry of any such importance as the ohm possesses ; moi-eover, it refers 

 expHcitly to rather a special thing, viz., a coil of wire, and that under 

 specified conditions, if it contain iron ; hence it would be rather absurd to 

 name this alone of all magnetic units. In the above communication, in 

 addition to a certain mode of defining the henry, it is u)-ged that unit 

 total induction be named too \ for this is the quantity which is of real 

 engineering importance — this is the quantity to attain which field-magnets 

 are built, and in the midst of which armatures are spun. 



It is also urged that it would be convenient if unit magnetic potential 

 could likewise be named, since electrical engineers have shown that they 

 have need of some such unit for the exciting cause of induction, by their 

 practical employment of the phrase ' ampere-turns.' The introduction of 

 a gauss unit, in some form not too obviously limited to the case of a wire- 

 wound coil, would assist teaching and would clarify magnetic ideas. 



The present writer does not presume to decide between the two 

 alternative systems of defining 'the gauss' as given above : viz., the 

 c.g.s. unit on the one hand, and the ampere-turn on the other. 



Oliver J. Lodge. 

 Liverpool: Dccemher 9, 1891. ' 



Postscript. — Another subject for discussion is whetlier L had better not 

 be defined as dN /dC ; with permeability as ^/=f^B/f?H to correspond. 

 This would make the three equations of page 203 stand thus : — 



(1) N= iLiC 



(2) E=LC 



(3) W=CN 



A letter just received from Mr. Heaviside indicates that lie would pro- 

 bably favour this course, and there is evidently much to be said for it. 

 I need hardly add that he contemns my temporising method of dealing 

 with the 4- nuisance. 



It need hardly be said that in the last resort it rests with practical 

 men to employ or decline any suggested system of units. Those who daily 

 deal with the quantities under consideration are the best judges of the 

 utility or otherwise of a suggested unit, provided always that they take 

 the trouble to give it a fair trial, and see how it works in practice. It 

 may be hoped that the above or similar suggestions will meet with criti- 

 cism at the hands of such men, and in order to make a beginning of 

 criticism I asked the Departmental Lecturer on Electrotechnics at Uni- 

 versity College, Liverpool (Mr. F. G. Baily), to consider them with special 

 reference to 



(1) The large size of the weber and henry units ; 



(2) the handiest definition of the gauss ; and 



(3) the least troublesome mode of bringing in the 47r. 



His reply, which is annexed, covers these points, and also incidentally 

 refers to the quantity called I or intensity of magnetisation. 



Now, as must often have been pointed out, the equation B:=H-F47rI is 

 a barbarous one, involving as it does quantities of diflTerent dimensions in 

 one equation. Its true meaning is of course B:=/xoH+ (^t — jko)H ; which, 

 although algebraically only a roundabout method of writing B=:^/H, is yet 



