PHYSICS. 371 



reut gradually increased from zero to the maximum gradually diminished 

 to zero again, and the same process repeated with a negative current. 

 It was found that the charcoal iron had the highest uiagnetizability, and 

 the soft steel the lowest; that of the soft Scotch iron approaching the 

 former. As to retentiveness, on the other hand, the charcoal iron shows 

 the least and the soft steel the greatest. Annealing the latter brings 

 it very near the common wire in respect to magnetizability and reten- 

 tiveness both. The cast iron specimens differ considerably also; the 

 malleable bar shows a much higher magnetizability than the cast-iron 

 ores, but its residual magnetization was so low that it could not be 

 measured. (Nature, March, 1886, xxxiii, 473.) 



In a series of experiments made to test the Ampere- Weber theory of 

 magnetism, Lodge subjected various substances to the action of a mag- 

 netic field, first strong to magnetize it and then weak and reversible to 

 test the magnetization produced. His observations have led him to the 

 conclusion that most likely every substance possesses some trace of 

 permanent magnetizability or retentivity. But in all these experiments 

 there is a flaw ; and that is that there is no guaranty that no trace of iron 

 is present in the body operated on. (Nature, March, 1886, xxxiii, 484.) 



Bidwell has studied the changes prodiiced by magnetization in the 

 length of iron wires under tension by means of a special apparatus so 

 arranged that the effects of magnetization on rods and wires can be ob- 

 served while they are under traction. Four specimens of iron were 

 used — a wire of commercial iron 1.2""" in diameter, a strip of annealed 

 charcoal iron, a piece of hard unannealed wire, and a wire of very pure 

 soft iron. The results show (1) that under the influence of a gradually 

 increasing magnetizing force the wire is at first elongated, then it re- 

 turns to its original length, and finally it contracts ; (2) that the max- 

 imum elongation diminishes as the load increases, but according to a 

 different law for each quality of iron ; (3) that the retraction due to a 

 given magnetizing force is greater with heavy than with light loads ; 



(4) that both maximum, elongation, and neutrality occur with smaller 

 magnetizing currents when the load is heavy than when it is light ; and 



(5) that the phenomena are greater for thin than for thick wires. 

 (Nature, April, 1886, xxxiii, 597.) 



The question of the influence of temperature upon magnetization has 

 been examined by Berson. He concludes from the experiments of him- 

 self and others that iron, steel, nickel, and cobalt behave when mag- 

 netized in the same general way in regard to temperature. At first 

 these metals take a magnetic moment, increasing with the temperature, 

 pass through a maximum limit, and then decrease to zero. The only 

 differences are (1) in the precise temperature corresponding to the max- 

 imum (about 220° for nickel), and (2) in the temperature at which the 

 magnetic power disappears, 340° for nickel, cherry-red for iron and steel, 

 the point of fusion of copper for cobalt. (J. Phys., October, 1886, II, 

 V, 4.?7.^ 



