Stress and Strain on the Properties of Matter. 



513 



Experiment V. 



The strips were now dismounted and the same one as had been 

 used before was placed in the axis of a magnetising helix,* especially 

 designed to prevent the heat of the helix from reaching the metal to 

 be magnetised. The two ends of the strip, which projected about 

 8 inches from either end of the helix, were connected with the galvano- 

 meter, and the clamp before used was now at one end of the helix and 

 just outside. The temperature was raised to about 60° C, and as 

 soon as the galvanometer needle had become fairly steady the helix 

 was excited by a single Leclanche cell.f A deflection of the galvano- 

 meter needle at once indicated a current from unmagnetised cobalt to 

 magnetised cobalt through the hot junction, and proved that longi- 

 tudinally magnetised cobalt is negative to unmagnetised cobalt. The 

 unannealed cobalt, therefore, under magnetic stress, behaves thermo- 

 electrically in a manner similar to iron. The helix in this experi- 

 ment was too far from the galvanometer to affect the latter directly, 

 and a reversal of the magnetising current produced a thermo-electric 

 current in the same direction as before. The magnetising force was 

 in this case equal to 47rx 906 x 0*478 x C in absolute units, where C 

 expresses the magnetising current in absolute units. The electro- 

 motive force of the cell was 1*5 volts very nearly, and the resistance 



1*5 x 10 8 



m circuit 1*8 ohms, consequently the value of C would be — -= — r— 5 

 H J 1-8 Xl0 9 



absolute units, and the magnetising force 43' 9 absolute units. 



This last result has an important bearing on the question whether 

 Bidwell's explanation of Hall's phenomenon is correct, because, 

 though unmagnetised cobalt is certainly rendered thermo-electrically 

 positive by longitudinal mechanical stress, the case may be different 

 when the metal is magnetised, as in Hall's experiment. J 



Nor indeed would it be safe to assume, without further experiment, 

 that mechanical stress will produce the same effect either in nature 

 or amount on the electrical resistance § or on the thermo-electric pro- 

 perties of magnetised iron, nickel, and cobalt, as it does on the same 

 metals when not under the influence of magnetic stress. 



Experiment VI. 



The strip of cobalt used in Experiments IV and V having been 

 annealed, was tested as before for the effect of mechanical stress on 



* For a description of this helix, designated as " the coil B," see loc. cit., p. 136. 



f One of the more recent kind, and which gives a fairly constant current. 



X See Sir W. Thomson's paper on " The Effects of Stress on the Magnetisation 

 of Iron, Nickel, and Cobalt," " Phil. Trans.," vol. 170, 1879. 



§ I may not, therefore, be right in my conjecture (see " Note on Hall's Pheno- 

 menon," " Phil. Mag.," May 1884, p. 402) that in all probability the " Hall 

 effect" on nickel will be diminished by raising the tempex*ature to 100° C. 



