M. Melloni on a new Electroscope. 277 



glass tube, with a ball or disc of metal E. B is a second reversed 

 metallic cup, a little smaller and much lighter than A ; it is 

 attached below a wire or very thin metallic lever CC, which is 

 suspended by its middle with a silk thread F. The axes of the 

 two cups are brought to the same vertical line, and the suspend- 

 ing thread, placed at such a level that the second cup is entirely 

 contained within the first, where it must be able to turn freely 

 round its point of suspension, without touching the walls of the 

 fixed cup A*. 



When the apparatus is thus arranged, it is clear that if the 

 conductor E receives an electrical charge, it will be propagated 

 by transmission to the outer cup A, and that when there it will 

 act by induction upon the inner cup B. If we suppose, for 

 example, that the electricity communicated is positive, this elec- 

 trical force distributed in A will disturb the equilibrium of the 

 natural fluid of B; will repel the positive fluid and attract 

 the negative, which Mill react in its turn upon the free fluid 

 of A, mask a certain quantity of it, and abandon the rest 

 to the known laws of electrical distribution on insulated con- 

 ductors ; so that the intensity of the action will depend on the 

 curvature of the surfaces, and will be weaker on the walls of the 

 cup than upon its appendages. The outer cup A of the charged 

 apparatus will consequently contain a certain proportion of 

 masked positive electricity (that is to say accumulated without 

 tension and without mobility), and its appendages DD will pos- 

 sess a free electricity of the same kind, which will increase in 

 energy in proportion as we approach the extremities. 



As regards the inner mass B and its lever CC, there will 

 be masked negative electricity on its central portion opposite 

 the cup A, and free positive electricity on the remainder of the 

 moveable system, that is to say, on the flat top of the reversed 

 cup and on the lever above it. Now the latter description of 

 electricity will evidently be far more energetic at the extremities 

 of the lever than at its middle portion or on the top of the cup, 

 — 1st, because these extremities constitute the most distant 

 points from the inductive action ; and 2nd, because the curva- 

 ture here is greater than anywhere else. 



Thus the lever CC, possessing the same kind of electricity as 

 the appendages DD, and being from its concentric position 

 subject to the mutual action of their repulsive forces, will be 

 energetically repelled, unless it be exactly in the same azimuth 

 with them ; after a few oscillations it will stop at a certain angle 



* In the electroscope which has been constructed, there is a peculiarity 

 not mentioned in the description, — a small metallic cylinder/ rises from 

 the middle of the fixed cup A; when the moveable cup is well hung, this 

 ryluuler occupies its interior without touching it. 



