Brown — Mechanical Stress and Magnetisation of Iron. l2l 



tlieu diminishes as the surroundiug field increases, and even reverses if the 

 field is sufficiently increased, as w&s shown by Villari.' In the former case 

 it would seem from these experiments that when the longitudinal magnetic 

 field and the current density in the wire are kept constant, and the cross- 

 sectional area of the wire varied, the maximum twist attains its highest value 

 with a wire of cross-sectional area 206 x 10"^ sq. cms. 



Conclusions. 



Tlie following conclusions may be arrived at from these experiments : — 



1. The momentary electric current produced by an axial twisting in a 

 magnetic field of an iron wire which is fixed at the top and in a vertical 

 position with a vibrator on its lower end, is very approximately proportional to 

 the amplitude of oscillation of the vibrator within tlie elastic limit of the wire. 



2. When the longitudinal magnetism and the cross-sectional area of an 

 iron wire are kept constant, the circular magnetism produced by twisting the 

 wire is diminished as the load is increased up to the -^-gth. part of the tenacity 

 of the wire. When the load is increased 3^ times, the circular magnetism is 

 decreased about 40 per cent. 



3. When the load and the longitudinal magnetism on an iron wire are 

 kept constant, the circular magnetism produced by twisting is proportional 

 to the cross-sectional area of the wire within the elastic limit of the wire. 



4. When the longitudinal magnetism is kept constant round an iron wire, 

 which is carrying an electric current of density 100 amperes per sq. cm., the 

 maximum twist of the free end of the wire is decreased 16 per cent, for an 

 increase of 70 per cent, in the load ; and for the same change in the load the 

 circular magnetism is decreased 9 per cent. 



5. When the load is kept constant on an iron wire which cai'ries a current 

 of density 100 amperes per sq. cm., the maximum twist of the free end of 

 the wire decreases as the longitudinal magnetism increases. For a load of 

 10* grammes per sq. cm. the twist is decreased 23 per cent, when the magnetic 

 field is increased 2*5 times; and when the load on the wire is increased 2"43 

 times, the maximum twist is decreased 30 per cent, for the same variation of 

 the longitudinal magnetism. 



6. When the load on an iron wire is kept constant at 10^ grammes per 

 sq. cm., and the longitudinal magnetism increased from 4 to 13'5 units, or 

 about three times, the circular magnetism is decreased about 50 per cent. ; 



' i'ogg. Ann., 18(58. 



