290 Mr. J. N. Heartier on the Amount of Electricity developed 



The second value (Q), which is negative, corresponds to the 

 second centre D' of maximum percussion ; it is situated on the 

 other side of, and at the same distance from, the spontaneous 

 centre : it gives a percussion opposite in direction to the im- 

 pulsion P which animates the body, and always less than the 



[To be continued.] 



XXXVI. On the Difference in the Amount of Electricity developed 

 by equal surfaces of Cylinder and Plate Electrical Machines. 

 By Jonathan N. Hearder, Mechanician, Plymouth"^. 



IT is a fact worthy of notice, that, whilst there has been a con- 

 tinual succession of important improvements in the modes 

 of developing electricity by chemical or voltaic action, as well as 

 in the apparatus for applying the electricity thus obtained to 

 practical purposes, the machuies for exciting Franklinian or 

 frictional electricity have remained, so far as the principles of 

 their construction are concerned, just in statu quo dm-ing the 

 last half century. It is true that machines have been constructed 

 of greater magnitude than formerly, but no attempt has been 

 made to examine the conditions upon which electrical excitation 

 depends, or the I'esults arising out of modifications of these con- 

 ditions. The electrical machines of the present day do not de- 

 velope more electricity from the friction of a given amount of 

 surface than those constructed fifty years ago; and indeed, from 

 occasional allusions made to the power of certain electrical ma- 

 chines in the various valuable scientific papers which I have 

 lately met with, I question much if the electrical machines of 

 the present day are at all equal to some constructed by Nairne, 

 Cuthbertson, Adams, and others. The rationale of the action 

 of the amalgam still remains unexplained ; and indeed so little 

 appears to have been done with regard to the investigation of 

 the action of the electrical machine, that I never remember to 

 have heard any notice taken of the extraordinary difference in 

 the amount of electricity obtained from the friction of equal sur- 

 faces on cylinder and plate electrical machines, the superiority 

 of the former being greater than as 4 to 1. I noticed this extra- 

 ordinary difference about twenty-six years since, when I had 

 frequent opportunities of contrasting one of my cylinder machines 

 of IQi inches in diameter with an excellent 3-feet plate machine 

 contrived by Sir W. S. Harris, and described in the Electrical 

 Number of Weale's Series, Transactions of the Plymouth Insti- 

 tution, and elsewhere. Although I did not then enter into any 

 very accurate examination of their relative powers, I was never- 



* Communicated by the Author ; Laving been read at the Plymouth In- 

 stitution and Devon and Cornwall Natural History Society, March 1, 1 858. 



