1902.] The Dissipation of Energy oy Electric Currents. 359 



J( The Dissipation of Energy by Electric Currents induced in an 

 Iron Cylinder when rotated in a Magnetic Field." By Ernest 

 Wilson, Professor of Electrical Engineering, King's College, 

 London. Communicated by Sir W. H. Preece, E.E.S. Ke- 

 ceived May 28,— Eead June 12, 1902. 



The effect which induced electric currents have upon the distribution 

 of magnetism in an iron cylinder, when rotated in a magnetic field, 

 has formed the subject of a communication,* and it is the object of this 

 paper to deal with the energy dissipated by these electric currents.! 

 A comparison will be made between the results of experiment and 

 theory. In connection with the latter, I have to thank J. B. Dale, 

 Esq., M.A., Assistant Lecturer in Mathematics, King's College, 

 London, for a contribution which at his wish I append to this paper. 

 The cylinder used in the experiments has diameter and length each 

 10 inches (25 -4 cm.). Since the previous experiments were made 

 ^additional holes have been drilled in a plane containing the longitudinal 

 -axis of the cylinder, and its present section is shown in fig. 1. 

 Insulated copper conductors have been threaded through these holes, 

 .and inclose areas indicated by the thick lines in fig. 1. The coils are 

 numbered 1, 2', and 4, and inclose areas of 25*8, 232, and 654 sq. cm. 

 respectively. As in the previous experiments, the electromotive force 

 in each coil has been calculated from the observed deflection of the 

 needle of a D'Arsonval galvanometer placed in each circuit. The 

 deflections were observed simultaneously, thus enabling the relative 

 phase-displacements to be determined. 



Specific Resistance. 



The specific resistance of the material of the cylinder has not been 

 directly determined. The late Dr. John Hopkinson gives 10*5 x 10 -<3 

 -ohm for mild steel, % and I have used 10 x 10 ~ ohm in connection 

 with the cylinder. A remarkable case of high specific resistance, com- 

 bined with high magnetic permeability and relatively small dissipation 

 -of energy by magnetic hysteresis under alternating magnetic force, is 

 .an alloy of iron and aluminium,§ in which the specific resistance is 

 39 x 10 ~ 6 ohm. Provided it keeps its good qualities after prolonged 

 ;use, this alloy should prove of great value. 



. * See Wilson, ' Roy. Soc. Proc.,' vol. 69, p. 435. 



f I wish, again to acknowledge the grant for the purposes of this research which, 

 was voted to me by the Council of the Royal Society out of the Grovernment Grant. 



% See ' Phil. Trans.,' 1885. 



§ See Barrett, Brown, and Hadfield, e Inst. Elec. Eng. Journ.,' vol. 31, part 4, 

 pp. 681 and 709. 



