12 Mr. Faraday and Dr. P. lliess on the Action of 



I thought it the more, as I saw no other waj^ to account for 

 this fact accordiug to your theory. When the rubbed shell-lac 

 cylinder is replaced by an electrified metallic globe, and a suffi- 

 ciently large metallic plate is placed at some distance above the 

 globe, the carrier receives only a weak charge from the centre of 

 the upper surface of the plate (which is not in sight of the globe), 

 and an increasing charge if it is raised. If the plate is insulated, 

 the carrier must not be applied to the plate, but the charge in- 

 creasing with the elevation of the carrier above the centre, and 

 the maximum of it in a certain height, is still remarked. Hence 

 I conjectured tliat you would consider the action of a metallic 

 intermediate plate between an inductric and an inducteous body 

 as screening the latter from the induction in straight lines of the 

 former*; and I was confirmed in my conjecture by § ]681t, 

 where you say " that the electric power is limited and exclusive." 

 Surely you will find this conjecture not to be an unfounded one, 

 if you call to mind that the philosophers who have adopted your 

 views on induction have made the same. Melloni has believed 

 to screen his electroscopes from the induction of a conductor by 

 the interposition of a metallic plate between both ; and De la Rive 

 relates with the same meaning the experiments with your differ- 

 ential-inductometer in his Traite d'Electricite, vol. i. p. 131 (of 

 which an English edition exists, which I have not seen). He 

 says, " Si on interpose une lame metallique soit isolee, soit 

 mieux encore, communiquant avec le sol, entre A (the positively 

 electrified inductric plate) et B (the inducteous, which has been 

 touched before), aussitot B donne des signes d'electricite negative 

 tres forte, qui pro\'iennent de ce que ['induction cessant d'agir 

 sur elle, etc. Ainsi, mettre un disque metallique entre A et B, 

 cela revient a remplacer B par uu autre disque plus rapproche de 

 A qu'il ne Fetait, et par consequent le soustraire a I'induction 

 deA." 



The metallic intermediate plate, insulated or not, is here said 

 to have withdrawn a body from the induction ; it is regarded as 

 a screen which intercepts the electric induction; as an opake 

 body intercepts the light. I am extremely gratified that you 

 partake not of this view, but I must confess that I cannot see 

 how to account, by the manner exposed in your letter, for the 

 results which I have obtained with intermediate conducting 

 plates. Let P be the originally electrified globe, N the unin- 



* If uninsulated, yes ; if insulated, no ; — as regards the final result of all 

 the actions (inductive and conductive) on the inducteous bodj'. — M. F. 



t "(1681.) A striking character of the electric power is that it is 

 limited and exclusive, and that the two forces being always present, are 

 exactly equal in amount. The forces are related in one of two ways : either 

 as in the natural normal condition of an uncharged insulated conductor, or 

 as in the chaiged state, the latter being a case of induction."— M. F. 



