MAGNETIC REACTIONS 41 



running, a single turn of wire was wrapped around one of the magnet coils 

 and connected to a ballistic galvanometer. The throw was measured 

 when the field current was turned on, and again when the disk was sud- 

 denly set in rotation. The latter throw was always in the opposite direc- 

 tion to the former; its measured value was certainly somewhat too small, 

 since it took an appreciable time for the disk to attain full speed. The 

 results indicated a diminution of the total flux amounting to only about 

 4 per cent, when the disk rotated at 320 revolutions per minute. Even 

 allowing for the gradual acceleration of the disk, it is apparent that the 

 reaction of the eddy currents causes chiefly an increased magnetic leak- 

 age, without greatly diminishing the flux through the coils. 



The diminution of the flux on starting the disk causes a slight momen- 

 tary increase in the current through the electro-magnet, while suddenly 

 stopping the disk diminishes the magnetizing current for an instant. 

 This is analogous to the momentary changes produced in the current 

 through a coil of wire when an iron core is moved in and out. Soret 1 

 seems to have observed this effect first. On the other hand, Jacobi 2 

 asserted that the magnetizing current was diminished when the angular 

 velocity of his disk was increased. If we understand his paper aright, 

 this must have been an error. 



(V) Effect of eddy currents on permanent magnets. It is of interest 

 to consider briefly the effect of moving masses of metal on permanent 

 magnets. If the pole of a bar magnet is held close to a rapidly revolv- 

 ing copper disk, its moment is permanently weakened. This method is 

 sometimes made use of in the artificial seasoning of horseshoe magnets. 

 In the design of at least one type of speedometer, this demagnetizing 

 action is especially guarded against in an ingenious manner. 



If one of the magnet systems of a Kelvin galvanometer employing 

 astatic needles is inclosed in a copper damper, this system undergoes a 

 slight demagnetizing action at every swing. Thus in time the astaticism 

 of the systems must be perceptibly impaired, unless the needles are very 

 well hardened. 



The currents induced in masses of metal moving relatively to per- 

 manent magnets must, at the beginning and end of the motion, induce 

 eddy currents in the magnet itself. If the acceleration is the same on 

 starting and stopping, these currents can have little to do with the demag- 

 netization of the magnet, for they flow in a direction tending to increase 

 the magnetization when the motion begins, and tending to decrease it 

 when the motion ceases. The case is analogous to moving the keeper 

 of a horseshoe magnet rapidly up against the poles, which causes de- 

 magnetizing eddy currents to flow, while suddenly pulling off the keeper 

 gives rise to currents in the opposite direction. 



1 Soret, Comptes rendus, 1857, 45, p. 301. 2 Jacobi, Comptes rendus, 1873, 74, p. 237. 



