1892.] Measurement of the Magnetic Properties of Iron. 503 



tabulated in the fourth column, it appears that the effect of the 

 current is to increase the retraction very slightly. 



According to Rowland the susceptibility of cobalt is increased by 

 ^heating. The small additional retraction indicated when the current 

 was passing was, therefore, no doubt due to the increased suscepti- 

 bility consequent upon current heating. It may be noted that 

 tension seems to have no material effect upon the magnetic retraction 

 of cobalt.* 



Summary. 



In an iron wire carrying a current, the maximum magnetic elonga- 

 tion is greater, and the retraction in strong fields is less, than when 

 no current is passing. The effect of the current is opposite to that 

 of tension. 



The magnetic retractions of nickel and of cobalt are not sensibly 

 affected by the passage of a current through the metals. (Tension 

 considerably modifies the magnetic retraction of nickel, but not that 

 of cobalt.) 



III. <; On the Measurement of the Magnetic Properties of Iron." 

 By Thomas Gray, B.Sc, F.R.S.E. Communicated by Lord 

 Kelvin, P.R.S. Received May 3, 1892. 



(Abstract.) 



This paper gives the method of experiment and results obtained in 

 some investigations on the time-rate of rise of current in a circuit 

 having large electromagnetic inertia. The experiments were made 

 on a circuit containing the coils of a large electromagnet having 

 laminated cores and pole pieces. The mean length of the iron 

 circuit was about 250 cm. and its cross section 320 sq. cm. The 

 magnetising coil had 3840 turns, when all joined in series, and a 

 resistance of 10'4 ohms. The coils were so arranged that they could 

 be joined in a variety of ways so as to vary the resistance, inductive 

 coefficient, &c, and also to allow the magnet to be used either as an 

 open or a closed circuit transformer. 



The electromotive force used in the experiments was obtained from 

 a storage battery, and the method of experiment was to trace the 

 curve, giving the relation of current to time, on a chronograph 

 sheet. 



One set of experiments shows the effect of varying the impressed 

 E.M.F. on the time required for the current to attain any given per- 

 centage of its maximum strength. The results show that for any 

 particular percentage there is always a particular E.M.F. which takes 



* Zioc. cit. 



