Eutherfokd. — On the Magnetization of Iron. 483 



A solenoid was wound on a small glass tube, sixty turns of 

 wire, seven turns to the centimetre. A leyden-jar, charged up 

 to a convenient potential by a Voss machine, discharged 

 through this solenoid, and any iron, whether solid or finely 

 divided, placed inside the solenoid was always more or less 

 magnetized by the discharge. 



Plate XLVIIL, Fig. 1. 



C, ordinary leyden-jar ; A is solenoid ; S, spark-gap. 



The whole of the discharge passed through solenoid A. 

 After the discharge had passed the needles were examined by 

 means of a small mirror magnetometer. As this magnetometer 

 is used in all future experiments for testing the magnetization 

 of needles, the construction is briefly explained. It was made 

 on the pattern set forth in Gray's " Absolute Measurements," 

 vol. ii., p. 79. The needle was small, and arranged in a 

 cavity, so that it was nearly dead-beat. The deflection was 

 increased by means of a lamp and scale in the ordinary way. 

 The value of the horizontal component at the needle was - 22, 

 and remained practically constant, as there were no masses of 

 iron in the vicinity. 



It was first settled that the needle placed in the solenoid 

 was unaffected by the charging current from the Voss. The 

 Voss was turned so as to charge up the jars just below the 

 potential necessary to spark across knobs at S. The needle 

 \\ as then removed and tested by the magnetometer. No 

 effect was observed. 



The effect of discharges on needles of different diameters 

 was first investigated. Length of needles, 7cm. : — 



(1.) Part of steel knitting-needle, diameter O103in.: De- 

 flection 112, at 9cm. distance from magnetometer needle. 



(2.) Pianoforte steel wire, diameter O032in.: Deflection, 

 40 ; distance from magnetometer, 9cm. 



(3.) Thin steel wire, diameter O008in. : Deflection, 10 ; 

 distance from magnetometer, 9cm. 



Diameter. Deflection. 



0T03in. ... ... ... ... 112 



0-032in. ... ... ... ... 40 



O'OOSin. ... ... ... ... 10 



It will be observed from these experiments that the deflec- 

 tion is very nearly proportional to the diameter of the wire. 

 This is to be expected, as the magnetizing forces are confined 

 to a thin skin of the substance. The amount of the magnetiza- 

 tion of the wire is proportional to the surface of the iron, and 

 not to its sectional area, as it is for steady currents. In 

 order to show that the effect was a surface one, and did not 



