ON CERTAIN THERMOELECTRIC EFFECTS OF STRESS IN IRON. 345 



It wu.s deteriniucd to experinient tir.st with injii wire. Previous 

 worker« liad all Ibiind that the thernioelectric effects of stress were 

 much more prouounced in this metal than in others. It seemed 

 natural therefore to begin with it. Sh(3uld the experiments prove 

 promising', it was intended to pursue the en(|uiry in regard to copper, 

 nickel, platinum, etc. A few experiments were indeed tried with 

 copper and nickel wires ; but in the latter its viscosity under the 

 influence of sustained stress pn^duced a gradual decay in the value of 

 the stress, applied as it was by a tightened screw. It was obvious 

 that a steady stress could be applied only l)y means of a load acting 

 by its weight ; and ïov this the apparatus was not readily adjustable. 



Several modifications in the mode of experimenting were repeated- 

 ly tried before resiüts (jf a satisfactory character were obtained. In cer- 

 tain experiments with the iron Avire, theruK^electric changes of very 

 small amount were (jbtained by simply varying the tension without 

 having established the temperature gradient. This thermoelectric 

 éUect increased with the tension. The direction of the current was 

 opposite to the direction of all the currents obtained when the gradient 

 of temperature existed ah^ng the wire. In (jther W()rds, the current 

 was such as miu'lit have resulted from a slight heating of the wire 

 where it was gripped by the dynamometer clamp. The probable 

 explanation of this effect is that the part of the stretched wire which 

 lay outside the tube was a little warmer than the part inside the tube. 

 ISuch a slight gradient of temperature might easily ensue under the 

 influence of the air as it grew warmer with the advance of dav, the 

 more massive tube changin£>' more slowly in tem])erature. If this is 

 the true explanation, the elfect will have no existence in the real 

 experiment, in which a steady temperature gradient is to be sustained. 

 In any case, however, these initial currents, as they might be termed, 

 were much smaller than the currents sul^secpiently oljtained. 



