82 Profs. J. A. Fleming and J. Dewar. On the 



Experiments on Annealed Swedish Iron. 



A cylinder of iron was formed by winding up a sheet of Saukey's 

 best transformer iron (Swedish).* The width of the strip was 

 4*895 cm., the thickness O0356 cm. ; three complete layers of the 

 sheet iron were used in forming the core. The area of cross-section 

 of the side of the cylinder so formed was 0'5229 sq. cm. The mean 

 diameter of the cylinder was 3*612 cm. This cylinder of iron was 

 placed in a clay crucible packed with magnesia, the lid luted on with 

 fire-clay, and the crucible then raised to a bright red heat in a forge, 

 after which it was allowed to cool very slowly. The iron cylinder was 

 thus carefully annealed out of contact with air or any material con- 

 taining carbon. This soft annealed iron ring was then wound over 

 with silk ribbon, and two windings of silk-covered copper wire placed 

 upon it ; the first or primary circuit consisted of 131 turns of No. 26 

 double silk-covered wire ; the secondary circuit consisted of 112 turns 

 of No. 36 silk-covered copper wire. The magnetising force to which 

 the ring is subjected when a current is sent through the primary coil 

 is measured by the value of 4<7r/10 x the ampere-turns per unit of 

 length of the mean perimeter of the ring, and this, in the case of the 

 present ring, reduces to the number 14'507 times the ampere current. 

 The magnetising force in absolute units is therefore very closely 

 given by the number obtained by multiplying the current flowing 

 through the primary coil in amperes by !4'5. The resistance of the 

 primary coil at about 15 C. was 0'92 ohm, and the resistance of the 

 secondary at the same temperature 8'98 ohms. The secondary 

 circuit of this ring coil or transformer was then connected through 

 appropriate resistances with a ballistic galvanometer, having a 

 resistance of 18 ohms. The primary circuit was connected through 

 suitable resistances and a current reverser with a circuit of con- 

 stant potential. By these arrangements it was possible to reverse 

 a definite current passing through the primary coils, and by observ- 

 ing the throw produced by the ballistic galvanometer, to calculate 

 the induction in the iron core. The galvanometer was calibrated by 

 reversing a known current passing through a long solenoid, in the 

 centre of which was placed a secondary coil of known turns and 

 dimensions, which was always kept in series with the secondary coil 

 of the transformer. In this manner a series of observations was 

 taken with gradually increasing magnetising forces. Before com- 

 mencing each series of observations, the ring was carefully demagnet- 

 ised by passing through the primary coil an alternating current, 

 which was gradually reduced in strength to zero, the ring coil being 

 thus brought into a magnetically neutral condition. An increasing 



* This sheet iron was kindly given to us by Mr. K. Jenkins, to whom our thanks 

 are due. 



