Page:Proceedings of the Royal Society of London Vol 60.djvu/97

82 A cylinder of iron was formed by winding up a sheet of Sankey’s best transformer iron ('Swedish).* The width of the strip was 4’895 cm., the thickness 0-0356 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 containing carbon. This soft annealed iron ring was then wound over with silk ribbon, and two windings of silk-covered copper wire placed upon i t ; 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 47r/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 14"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 constant potential. By these arrangements it was possible to reverse a definite current passing through the primary coils, and by observing 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 commencing each series of observations, the ring was carefully demagnetised 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

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