on a new Eledro-dynmnic Experiinent. 375 



According to this disposition the whole of the voltaic cir- 

 cuit is composed : 



1. Of the arc BB' and the conductors MN, M'N'. 



2. Of a circuit formed by the parts RIP, P'I'R' of the ap- 

 paratus, by the curved conductor going from R' to S, and the 

 pile itself. 



This last circuit must act as a complete one, since it is not 

 interrupted except by the thickness of the glass which isolates 

 the two cups P, P': it will therefore be sufficient to observe its 

 action on the arc BB' in order to establish by experiment the 

 action of a complete circuit upon an arc in the different po- 

 sitions we may give it. 



WheUj by means of the joint O, the arc AA' is placed in 

 such a position that its centre is outside the axis G H, 

 this arc begins to move, and slides on the mercury of the 

 little troughs MM' by the force of the action of the complete 

 curved current, which runs from R' into S. If on the con- 

 trary its centre is in the axis, it remains immoveable : the com- 

 plete circuit has therefore no action to make it turn round the 

 axis, and that whatever be the size of the part BB' determined 

 by the open ing of the angle of the conductors MN, M'N'. If, 

 therefore, we take two arcs BB' differing little from each 

 other, as the momentum of rotation is null for either of them, 

 it will be null for their little difference, and therefore for every 

 element of the circumference, the centre of which is in the 

 axis ; whence it follows that the direction of action which the 

 complete circuit exercises on the element, passes through this 

 axis, and is thus perpendicular to the element. 



When the arc A A' is situated so that its centre is in the 

 axis, the portions of the conductors MN, M'N' exercise on the 

 arc BB' equal and opposite repulsive actions, in such a man- 

 ner that no effect can result from it ; and since there is no 

 motion, we are sure that there is no momentum of rotation 

 produced by the complete circuit. 



When the arc AA' moves in the other situation in which 

 we supposed it first, the actions of the conductors MN and 

 M'N' are no longer equal. One might be led to believe 

 that the motion is owing only to this difference; but in pro- 

 portion as we approach or remove the curved conductor 

 running from R' to S, the movement is increased or dimi- 

 nished ; which circumstance leaves no room for doubt that 

 the complete circuit bears a great share in the effect noticed. 



If we once establish by this experiment that the action of 

 a complete circuit on an element of the voltaic circuit is al- 

 ways perpendicular to the direction of this element, wc may, 

 by a very simple calculation, deduce from it the relation be- 

 tween 



