170 Mr. F. C. Webb on an Electrical Experiment 



sensitive when a variation in w, the external resistance belonging 

 to coil g, takes place. 



Now it is clear that equations (IL) and (II'.) are not necessarily 

 identical, as long as p does not fulfil certain conditions, and 

 therefore the first set of equations (II.) and (1) may give entirely 

 different values for g and g^ from those obtained from the second 

 set (IF.) and (I. ), which means that a simultaneous maximum sen- 

 sitiveness with respect to an alteration of the external resistance 

 w or iJ in either of the two differential branches, is not always pos- 

 sible. The following very important and interesting question, 

 therefore, remains to be solved. 



What general condition must he fulfilled in the construction of 

 any differential galvanometer in order to make a simultaneous maoci- 

 mum sensitiveness possible, with respect to an alteration of external 

 resistance in either of the differential branches ? 

 [To be continued.] 



XXII. On an Electrical Experiment with an Insulated Room, 

 By r. C. Webb, M. Inst. C.E.^ 



SOME ten years ago, in a series of articles in ' The Electri- 

 cian,^ afterwards published in book-form, I submitted 

 some new views on the explanation of the action which takes 

 place in various well-known electrical phenomena. I endea- 

 voured to prove by reasoning that the ordinary explanations 

 given of the discharge of a charged conductor by communica- 

 tion with the ground were erroneous ; and I suggested an ex- 

 periment with an insulated room as a means of proving the 

 fallacy of these ordinary explanations of discharge &c. This 

 experiment I performed in May 1869 at Keyham Dockyard ; and 

 I think the results are worth placing on record. 



To make the bearing of the experiment intelligible, however, 

 it is necessary to recapitulate a little of what I have before urged. 



It is generally stated in books on electricity, that, when an 

 electrical machine has its rubber to earth, on the action of the 

 machine the prime conductor becomes charged positively and 

 the equal quantity of negative electricity which is generated is 

 lost in the earth — and that when the prime conductor is then 

 placed in contact with earth, its discharge is merely the conse- 

 quence of its sharing its electricity with the earth, the propor- 

 tion remaining on the conductor being, in virtue of the almost 

 infinite size of the earth as compared with the conductor, almost 

 infinitely small and consequently quite inappreciable. 



I will not weary your readers by quoting, but will merely refer 

 them to: — Professor DanielFs ^Chemical Philosophy,'' p. 241 j 

 * Communicated by the Author. 



