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APPLICATION TO THE STUDY OF MAGNETISM. 541 



Instead of sending the alternating current into the external 

 circuit, a commutator may be placed on the axis of rotation, 

 consisting of two half discs communicating separately with the 

 ends of the frame, and of two springs connected with the external 

 circuit. If the springs are so adjusted as to change the connections 

 the moment the current is null that is to say, at a period <T 

 after the passage of the frame through a plane perpendicular to 

 the magnetic meridian the direction of the current in the external 

 circuit is always the same, and there is no change in the law 

 of the phenomena, as the commutator does not cause any loss 

 of energy. 



If we use the frame as a motor by connecting it with an external 

 electromotive force E , it will be necessary to use a commutator 

 which changes the direction of the current twice in each period, 

 in order to maintain the motion. There is then a loss of energy 

 by the sparks at the movable contacts which must be allowed for, 

 and the calculations are far less simple. 



It is impossible to treat here in greater detail the question 

 of induction-electromotors. We shall return to the subject in the 

 second part of this work. 



559. APPLICATION TO THE STUDY OF MAGNETISM. We are 

 now in a position to justify the method mentioned in (417) for 

 studying the distribution of magnetism, a method employed in 

 the experiments of Van Rees and of Gaugain. 



If a magnetised bar is surrounded at a point by a coil 

 formed of ri turns, connected with a galvanometer, and if the 

 coil is suddenly made to slide through a certain distance, parallel 

 to the bar, coming to rest in its fresh position, a certain quantity 

 of electricity m passes through the coil ; if R is the total resistance 



of the circuit, the expression r is equal to the flow of magnetic 



n 



force, which proceeds from the magnet between the two positions 

 of the coil. Working in this way by successive displacements, 

 we can determine the law of variation of the flow of the lateral 

 force. 



If the coil is at first in the middle of the magnet, or more 

 exactly at the neutral point, and it is suddenly removed to a great 

 distance, we shall get the total flow of force from the magnet, 

 and therefore the total mass of free magnetism contained in the 

 corresponding portion of the bar. 



If the auxiliary coil thus surrounds either the centre of a long 

 cylindrical coil, containing a bar of soft iron, or any point of an 



