Nickel begins to lose suddenly its Magnetic Properties. 375 



until the deflection of the Thomson-galvanometer, produced 

 by suddenly turning the earth-coil, was exactly 80 divisions 

 of the scale. The rheostat in the primary circuit was next 

 adjusted until the required current had been reached, care 

 being taken to begin with a small current, which was increased 

 very gradually to the required amount. As it was the aim of 

 the author to test only the temporary permeability * of the 

 nickel, the current in the primary circuit was opened and 

 closed a great many times, until the current induced on closing 

 the circuit became the same in magnitude as that induced on 

 opening the circuit ; and then the mean of the deflections 

 produced by ten times closing the circuit was taken to 

 measure the temporary induction. The air-chamber was 

 now allowed to cool a little, and then, as before, after the 

 temperature had been steadied by adjusting the burners, a 

 fresh set of observations was made, and so on until the tempe- 

 rature of the room was reached. As soon as the observations 

 with the lowest magnetizing force had been completed, fresh 

 ones were made with a higher magnetizing force until a mag- 

 netizing force of 18'183 C.G.S. units had been reached. 



The temporary permeability of the nickel was calculated 

 from the following formula? : — 



M,= *=£, (i) 



*-33c « 



In formula (1) My is the magnetizing force, n x is the num- 

 ber of turns in the primary coil, I the length of the coil, and 

 C the current circulating round the coil. 



In formula (2) M p is the magnetic permeability (ratio of 

 magnetic induction to magnetizing force), N" the number of 

 turns in the earth-coil, A the area of the earth-coil, D the 

 deflection produced by suddenly turning the earth-coil through 

 180°, V is the vertical component of the earth's magnetic 

 force, d the induction-current due to the nickel, S the section 

 of the nickel, and n 2 the number of turns in the secondary 

 coil. C.G.S. units were used throughout. Since the length 

 of the nickel under the magnetizing force was about 350 times 

 the diameter, the effect of the ends is quite negligible. The 



* It is impossible to obtain true relations between change of tempera- 

 ture and change of total permeability by the "ballistic method,'' because 

 the mere act of changing the temperature shakes out some of the sub- 

 permanent magnetism which the metal may have acquired at previous 

 temperatures. 



2 C 2 



