Iron under Alternating Magnetic Force. 



227 



induction density over its section. On account of the increased lag of 

 phase of induction as the centre is approached, the maximum average 

 over the whole core is not materially altered for the same surface 

 density. The force due to the current in the magnetising coils is 

 smaller at 53° C. for the same maximum average induction density 

 over the whole core. For a given permeability and hysteresis loss the 

 higher the specific resistance and temperature coefficient the better. 



It should be mentioned that the potential difference employed in 

 these experiments was 200 volts, the excess over the magnet and 

 liquid reverser being taken up by non-inductive resistance. The area 

 taken for each coil is the actual area of iron in the plane of section, 

 fig. 2. The areas taken for coils 1, 2, 3, and 4 are 19'8, 8'465, 19'8, 

 and 21*16 sq. cm. respectively. If, instead of these, we take the areas 

 bounded by the centre lines of the ^-inch holes, the diminution of 

 induction density would be 30*6, 52*4, 30*6, and 22 per cent, respec- 

 tively. The true correction will not alter the general conclusions 

 arrived at in the paper, and is a function of the permeability of the 

 iron. The figures in the table in italics are the result of taking the 

 increased areas, so that a comparison can be made. The D'Arsonval 

 galvanometers used have slightly different dead-beatness. The least 

 and most dead-beat instruments were placed in series in the No. 1 

 circuit, when the changes of E.M.F. were most rapid. The instruments 

 gave the same result within the limits of error in observation. A 

 variable still to be dealt with is the wave-form of the magnetising 

 currents. 



I wish to express my thanks to Mr. Wm, Harden for the assistance 

 he has given me in the work connected with this paper. Mr. F. S. 

 Robertson, Mr. Nunes, and Mr. Browne have also helped me. To 

 these gentlemen I tender my thanks. I have also to thank Messrs, 

 Elliott Bros, for the loan of three out of the five D'Arsonval galvano- 

 meters used in the experiments. The experiments were made at 

 King's College, London. 



