>88 ELECTRIC INDUCTION. 



5. Let UB begin by dividing the time into a num- 

 ber of equal intervals, each equal to St. The smaller 

 we take these intervals the more accurate will be the 

 definition of the train of images which we shall now 

 describe. 



6. At a given time t, let a positive image of the 

 magnet or electro-magnet be formed on the negative 

 side of the sheet. 



e 



As soon as it is formed, let this image begin to 

 move away from the sheet in the direction of the O ^ 



normal, with the velocity R, its form and intensity 

 remaining constantly the same as that which the 

 magnet had at the time t. 



After an interval 8t, that is to say, at the time t + St, let a negative 

 image, equal in magnitude and opposite in sign to this positive image, be formed 

 in the original position of the positive image, and let it then begin to move along 

 the normal, after the positive image, with the velocity R. The interval of time 

 between the arrival of these images at any point will be St, and the distance 

 between corresponding points will be RSt. 



7. Leaving this pair of images to pursue their endless journey, let us 

 attend to the real magnet, or electro-magnet, as it is at the tune t + St. At 

 this instant let a new positive image be formed of the magnet in its new 

 position, and let this image also travel in the direction of the normal with the 

 velocity R, and be followed after an interval of time St by a corresponding 

 negative image. Let these operations be repeated at equal intervals of time, 

 each of these intervals being equal to St. 



8. Thus at any given instant there will be a train or trail of images, 

 beginning with a single positive image, and followed by an endless succession of 

 pairs of images. This trail, when once formed, continues unchangeable in form 

 and intensity, and moves as a whole away from the conducting sheet with 

 the constant velocity R. 



9. If we now suppose the interval of time St to be diminished without 

 limit, and the train to be extended without limit in the negative direction, so 

 as to include all the images which have been formed in all past time, the 

 magnetic effect of this imaginary train at any point on the positive side of the 



