NATUEE 741 



hypothesis is not a necessity, for we shall presently see that Helmholtz 

 rejected it. However that may be, it enabled Ampere, although he 

 had never produced an open current, to lay down the laws of the 

 action of a closed current on an open current, or even on an element 

 of current. They are simple : 



(1) The force acting on an element of current is applied to that 

 element; it is normal to the element and to the magnetic force, and 

 proportional to that component of the magnetic force which is nor- 

 mal to the element. 



(2) The action of a closed solenoid on an element of current is 

 zero. But the electro-dynamic potential has disappeared i.e., when 

 a closed and an open current of constant intensities return to their 

 initial positions, the total work done is not zero. 



3. Continuous Rotations. The most remarkable electro- dynamical 

 experiments are those in which continuous rotations are produced, 

 and which are called unipolar induction experiments. A magnet may 

 turn about its axis; a current passes first through a fixed wire and 

 then enters the magnet by the pole N, for instance, passes through 

 half the magnet, and emerges by a sliding contact and re-enters the 

 fixed wire. The magnet then begins to rotate continuously. This is 

 Faraday's experiment. How is it possible? If it were a question of 

 two circuits of invariable form, C fixed and C' movable about an axis, 

 the latter would never take up a position of continuous rotation; in 

 fact, there is an electro-dynamical potential; there must therefore be 

 a position of equilibrium when the potential is a maximum. Con- 

 tinuous rotations are therefore possible only when the circuit C' is 

 composed of two parts one fixed, and the other movable about an 

 axis, as in the case of Faraday's experiment. Here again it is con- 

 venient to draw a distinction. The passage from the fixed to the 

 movable part, or vice versa, may take place either by simple contact, 

 the same point of the movable part remaining constantly in contact 

 with the same point of the fixed part, or by sliding contact, the 

 same point of the movable part coming successively into contact with 

 the different points of the fixed part. 



It is only in the second case that there can be continuous rotation. 

 This is what then happens : the system tends to take up a position 

 of equilibrium ; but, when at the point of reaching that position, the 

 sliding contact puts the moving part in contact with a fresh point in 

 the fixed part; it changes the connections and therefore the conditions 

 of equilibrium, so that as the position of equilibrium is ever eluding, 

 PO to speak, the system which is trying to reach it, rotation may take 

 place indefinitely. 



Ampere admits that the action of the circuit on the movable part 

 of C' is the same as if the fixed part of C' did not exist, and therefore 

 as if the current passing through the movable part were an open 



