1882.] Stress and Strain on the Action of Physical Forces. 283 



The alteration of electrical conductivity which can be produced by 

 magnetisation was carefully studied, and a full account of the modes 

 of experimenting, of the apparatus employed, and the precautions 

 adopted will be found in the paper. The substances examined were 

 iron, steel, nickel, cobalt, bismuth, copper, and zinc, and in all cases, 

 except that of copper, it was proved that longitudinal magnetisation 

 increases the electrical resistance, whether the substance is in an 

 annealed or unnannealed condition. With copper wire no trace of 

 change of resistance could be detected when the wire was under the 

 influence of a powerful electro-magnetic solenoid. In the case of zinc, 

 which was in the form of foil, no alteration of resistance was discern- 

 ible until the action of the solenoid was supplemented by that of a 

 stout iron core, which was placed inside the solenoid and round which 

 the foil was wrapped. In the next table will be found the increase of 

 resistance per unit produced in the different substances by an 

 absolute electro-magnetic unit of magnetising force, when the mag- 

 netisation is longitudinal. 



Name of metal. 



Condition. 



Diameter in 

 millimetres. 



Increase of re- 

 sistance per unit 



produced by- 

 unit magnetising 

 force. 





Annealed 



0-94 



2335 x 10- s 



Steel 



Annealed 



0-85 



1500 



Steel 



Unannealed .... 



2-33 



1137 



Steel 



Very hard 



2 33 



70 





Annealed 



1-05 



8070 



Nickel 



Unannealed .... 



7 -00 



4343 



Cobalt 



Unannealed .... 



7-50 



628 





Unannealed .... 



3-30 



21 



Of all the metals examined, annealed nickel was by far the most 

 affected by a given amount of magnetising force. 



The increase of resistance produced by magnetisation can be 

 very accurately represented by the formula 7=a . a, + b . ft ; where y is 

 the increase of resistance, cc the magnetising force, ft the induced 

 magnetism, and <z, b constants for the same substance when the same 

 amount of current per unit of area flows through the substance. 



When different strengths of current are used in measuring the 

 resistance of annealed iron, the alteration of resistance caused by a 

 given magnetising force increases with the amount of current per unit 

 of area which flows through the substance. The induction current 

 produced by the magnetisation of the iron in a coil surrounding the 

 latter is also greater when a current is flowing through the iron than 

 when it is not. With annealed nickel, or with unannealed steel, 



