Changing the Magnetic Moments of Steel Magnets. 297 



I = half the distance between the poles of the deflecting 

 magnet (in these experiments, taken as half the 

 actual length of the magnet) ; 

 H = the horizontal component of the Earth's magnetic 



force =-153 C.G.S. unit; 

 6 = the deflection, in degrees, of the magnetometer- 

 needle. 



In order to test the constancy of the magnetic field during 

 the experiments, the deflections given by a standard magnet 

 were occasionally taken. 



The method employed to obtain the effects of percussion, 

 with the least possible amount of handling of the magnets, 

 was as follows : — A series of glass tubes, wide enough inside 

 to allow the magnets to fall through them freely, were fixed 

 on a long narrow board by means of brass clamps, which were 

 just loose enough to allow the tubes to slip easily through them. 

 This was for the purpose of raising the tube vertically in order 

 to take the magnet out after falling through it. A thick shelf 

 was firmly fixed at one end of this board and a thick plate of 

 glass fastened to it. The magnets were held in the hand and 

 allowed to fall vertically through the tube upon the glass plate 

 at the bottom, and always with the true north end of the 

 magnet downwards. 



A few trial experiments were made in letting the magnets 

 fall through a height of a half metre and one metre respec- 

 tively ; but to give uniformity in the results, the 1*5 metre 

 height was adopted throughout. 



The plan of experimenting was as follows : — The magnets 

 were magnetized and laid aside undisturbed for the periods of 

 time specified in Table I. One of them was then taken, and 

 the deflection for calculating its magnetic moment was ob- 

 served. It was then allowed to fall once through the height 

 of 1*5 metre, and the deflection again taken. It was then let 

 fall three times in succession through the same height, and the 

 deflection again taken. Each magnet in turn was put through 

 the same series of operations. 



The percentage loss in the magnetic moment due to the 

 one fall, and that due to the three falls, and, finally, that due 

 to the whole four falls, were all calculated, and are shown for 

 the fourteen glass-hard magnets in Table II. 



In Table I. the percentage loss due to the four falls alone 

 is given. The magnets specified in Table II. had been lying 

 aside for a period of six months after being magnetized, and 

 before they were experimented on. 



