Butherford. — On the Magnetization of Iron. 487 



Now, — is the value of the tangent of the angle that the 

 tangent at any point of the curve B (see Plate XLVIIL, 



/■7TYT 



Fig. 3) makes with the axis of abscissae. -37- is .\ known from 



curve B. We can consequently determine the curve of varia- 

 tion of I from the surface to the centre, although there are 

 not sufficient data to actually calculate I in absolute measure. 



Plate XLVIIL, Fig. 4. 



The curve in Fig. 4 is an approximate representation of the 

 magnetization from the surface inwards. The ordinates repre- 

 sent I, the intensity of magnetization. The abscissas repre- 

 sent the distances from the external surface of wire. 



It will be observed that the surface-layer is magnetized in 

 an opposite direction to the main part of the magnetized 

 metal. 



As we go inwards from the surface the intensity of 

 magnetization rapidly decreases till at the point A there is a 

 portion of the metal which is not magnetized. This will be 

 called the " neutral point." 



On penetrating still further the magnetization changes 

 sign, and rapidly rises to a maximum, which most probably 

 represents an intensity corresponding to the saturation-point 

 of steel. The intensity then remains practically constant till 

 at D it decreases very rapidly down to zero. 



It is evident from the manner in which the magnetization 

 varies inwards that the iron has been under the influence of 

 an oscillatory discharge. The first half-oscillation penetrated 

 to a depth of 3-i^in., which is represented by the length O B 

 in the figure. The neutral point A is at a depth of about ^o^ n * 

 from the surface. The second half-oscillation has evidently 

 decreased in amplitude considerably, since the depth of pene- 

 tration is only a quarter that of the first discharge. 



In this experiment there is only evidence of two half- 

 oscillations. Several needles were examined which had been 

 magnetized under the influence of various fields and different 

 lengths of spark-gap, but the existence of the return oscilla- 

 tion could not with certainty be detected. 



All the needles used gave the same general result — viz., a 

 thin surface-layer magnetized in one direction, and a thicker 

 interior layer magnetized in the opposite direction. 



In one case examined the depth of penetration of the first 

 discharge was considerably less than xoVo^- 



The effect of varying the capacity of the condenser and 

 keeping the self-inductance and the spark-gap constant was 

 also investigated. 



