Euthebfokd. — On the Magnetization of Iron. 485 



A uniformly-magnetized steel needle was found to dissolve 

 very regularly till it was reduced to an extremely fine filament, 

 which did not break up until the magnetometer deflection 

 had fallen within 3 div. of zero. The following is the result 

 of an experiment on a uniformly-magnetized needle (needle 

 0'032in. in diameter ; steady deflection just before acid is 

 poured in = 222) : — 



Time in Seconds 

 after Solution begins 







30 



49 



56 



90 

 115 

 139 

 206 

 246 

 311 

 373 

 414 

 454 

 566 

 638 



Deflection. 



. 222 



. 217 



, 195 



. 177 



. 157 



. 147 



. 137 



. 107 



. 97 



. 77 



. 57 



. 47 



. 37 



. 17 

 7 



It would be expected that the rate of solution of the 

 metal at any instant would be : : al to the surface of the metal 

 at that instant — that is, to the radius of the wire. This is very 

 accurately the case in the above experiment. The deflec- 

 tion of the magnetometer at any instant is proportional to 

 the sectional area of the wire — i.e., to the square of the 

 radius. The radius of the wire at any moment is therefore 

 known. 



If a curve is constructed whose abscissae represent time, 

 and ordinates the radii of the wire at different intervals, it 

 will be found to be nearly a straight line, with the excep- 

 tion of an irregularity in the beginning of the curve. 



A needle of steel 0-032 in diameter was then taken, and 

 magnetized by passing the discharge from four leyden-jars in 

 parallel through the solenoid. Spark-length, ^in. A correc- 

 tion was made for the fall of deflection when needle was 

 immersed in dilute HN0 3 at 100° C. 



Plate XLVIIL, Fig. 3. 



The following is the table of observed values of time and 

 deflection. The values of time and deflection are reduced for 

 convenience in plotting curves : — 



