Bkown — Mechanical Stress and Magnetisation of Iron. 189 



magnetic field which produces maximum circular magnetisation 

 is different with different-sized wires. 

 2. When the longitudinal load is 10*^ grammes per sq. cm. and the cross- 

 sectional area of the wire is increased about 4"7 times : — 



(ff) The longitudinal magnetic field which produces the maxi- 

 mum circular magnetism is decreased about 54 per cent. 

 [b) The maximum circular magnetisation is increased 2 '5 times. 

 {c) Tlie longitudinal magnetic field required to give the highest 

 value of the maximum transitory current is decreased 

 28 per cent. 

 (rf) The highest value of the maximum transitory current is 

 increased 3"7 times. 

 B. When an electric current is sent througli tlie wire and the mechanical 

 twist measured : — 



1. The change in the mechanical twist lags behind tlie change in the 



circular magnetisation, a cyclic curve and hysteresis loops are 

 obtained. 



2. When an iron wire of cross-sectional area, 20'6 x 10"^ sq. cms., is 



placed in a uniform magnetic field of 2"5 c.g.s. units, and the 

 longitudinal stress per unit area on tlie wire is increased about 

 7 times, the circular magnetism is decreased about 30 per cent., 

 and the maximum twist of the free end of the wire is decreased 

 about 14 per cent. 



3. The value of the longitudinal magnetic field whicli produces maximum 



circular magnetisation is different with different-sized wires. 



4. When the longitudinal load is 10^ grammes per sq. cm. and the cross- 



sectional area of the wire is increased about 4*7 times: — 



(«) The longitudinal magnetic field which produces maximum 



circular magnetisation is decreased about 28 per cent. 

 {b) The maximum circular magnetisation is increased about 

 three times. 

 I am indebted to Mr. F. W. Warwick, b.a., b.e., of the Physics 

 Laboratory, Royal College of Science, for assistance in reading the gal- 

 vanometer scale during some of the experiments for both this and the 

 previous paper. 



