20 



MU. H. TOUfLlNSON ON THE INFLUENCE OP STRESS AND 



By subtracting '000845 from the other numbers in the same column we obtain the 

 increase of logarithmic decrement due to the reversals of the current in each case. 



The increase of the logarithmic decrement increases proportionately to the current 

 until the latter attains a value of 0'0616 ; from this point upwards the ratio B: A 

 increases. Since the circuit was broken immediately after each reversal the current 

 might have been earned much higher without sensibly heating the wire,* and, in all 

 probability, with circular magnetisation as with longitudinal magnetisation, the ratio 

 B : A would, beyond a certain value of the magnetising stress, begin to decrease as 

 the stress increased. There was, however, the following difficulty in experimenting 

 with higher currents : In consequence of the wire hanging vertically it was all the 

 time under the influence of the earth's vertical magnetic stress, and was thereby 

 longitudinally magnetised, so that when the current was sent up or down the wire 

 the combination of the two magnetisations caused the wire to twist to this side or 

 that ; and, since the reversals of the current were synchronous with the vibrations of 

 the wire, the amplitude of these last vibrations was, owing to the above-mentioned 

 cause, increased or diminished according as the phases of the two sets of vibrations 

 were the same or opposite. With the smaller currents the difficulty was sufficiently 

 overcome by first making a number of reversals with the two sets of vibrations in the 

 same phase, and then an equal number with the phases opposed ; but with the 

 current 0'205G this mode of compensation began to fail. It is evident, however, 

 that the internal friction of iron can be very largely increased by reversals of a 

 current passing through the wire.] 



The Effect of Magnetisation on tJie Longitudinal Elasticity. 



According to GUILLAUME WunTHEiMt long continued magnetisation diminishes both 

 temporarily and permanently the longitudinal elasticity of iron and steel, but 

 magnetisation continued only for a short time has no sensible effect. Since the 

 author's own experiments led him to view with considerable caution these results of 

 M.WjRTHEiM,and to believe that long continued magnetisation diminished the elasticity 



Rise of temperature causes decrease of the internal friction of annealed iron, 

 t ' Annales de Chimie,' vol. 12, 1844, p. 610. 



