390 ON MAGNETS. 



magnetic force, nor of the modifications produced by the presence of 

 soft iron in the magnetic state of the bar exactly in the region we 

 are exploring. The results furnished by the use of soft iron do not 

 then seem to be so definite as those obtained with magnets. 



417. Measurement of the Flow by Induction Currents. 

 This method is the only one which gives exact results ; the theory 

 will be given subsequently. It is sufficient here to remark, that by 

 means of induction currents we may determine the flow of force, or 

 the flow of magnetic induction across a closed circuit. 



If the bar is anywhere surrounded by a ring formed of one or 

 many turns and connected with a galvanometer, and if by any 

 method we suddenly suppress the magnetisation, the momentary 

 current produced in the ring measures the total flow of induction 

 which traverses the plane bounded by the ring at the point in 

 question ; if the ring clasps the bar tightly, the flow of induction 

 which traverses the ring is that which exists in the section of the 

 bar itself. 



The ring being placed in the same point it is caused to glide 

 along the axis of the bar to a distance which may be regarded as 

 infinite ; the current, in this case, measures the total flow of force 

 emanating from the magnet, measured from the point of departure. 



Experiment shows, as indeed is evident from the theorem of the 

 conservation of the flow of induction, that the current is the same as 

 in the preceding case. 



By measuring in either way the flow corresponding to different 

 points, we may construct a curve which represents the magnetic 

 condition of the bar. The curve has a maximum ordinate which 

 corresponds to the neutral line ; it sinks on each side and becomes 

 an asymptote to the axis of the bar, which we suppose to be pro- 

 longed indefinitely. This may be called with Gaugain the curve 

 of demagnetisation. 



If, while the ring is at a point, the abscissa of which is x, it is 

 displaced by a quantity dx, the current measures the external flow 

 corresponding to this length dx, or, what is the same thing, the 

 variation in the internal flow of induction. By successively displac- 

 ing the ring by equal amounts, we may construct the curve whose 

 ordinates represent the external flow, and therefore the perpendicular 

 component at the various points. The ordinates of this curve are 

 the differentials of the ordinates of the curve of demagnetisation. 



This method furnishes then, like the preceding, but in an exact 

 manner, the values of the perpendicular component at every point 

 of the bar. 



