96 CORRELATION OF PHYSICAL FORCES. 



nies the transmission of electricity: the wires are heated in a 

 degree decreasing as their thickness increases but by in 

 creasing the delicacy of our tests as the heating effects de 

 crease in intensity, we may indefinitely detect the augmenta 

 tion of temperature accompanying the passage of electri 

 city and wherever there is augmentation of temperature 

 there must be expansion or change of position of the mole 

 cules. 



Again, it has been observed that wires which have for a 

 long time transmitted electricity, such as those which have 

 served as conductors for atmospheric electricity, have their 

 texture changed, and are rendered brittle. In this observa 

 tion, however, though made by a skillful electrician, M. Pel 

 tier, the effects of exposure to the atmosphere, to changes of 

 temperature, &c., have not been sufficiently eliminated to 

 render it worthy of entire confidence. There are, however, 

 other experiments which show that the elasticity of metals is 

 changed by the passage through them of the electric current. 



Thus M. &quot;YVertheim has, from an elaborate series of ex 

 periments, arrived at the conclusion that there is a temporary 

 diminution in the coefficient of elasticity in wires while they 

 are transmitting the electric current, which is independent of 

 the heating effect of the current. 



M. Dufour has made a considerable number of experi 

 ments with the view of ascertaining if any permanent change 

 in metals is effected by electrisation. He arrives at the cu 

 rious result that in a copper wire through which a feeble vol 

 taic current has passed for several days, a notable diminution 

 in tenacity takes place ; while, in an iron wire, the tenacity 

 is increased ; and that these effects were more perceptible 

 when the wires had been electrised for a long time (nineteen 

 days) than for a short time (four days). The copper wire 

 was, in his experiment, not perfectly pure ; so that the effect, 

 or a portion of it, might be due to the state of alloy : in the 

 case of iron, the magnetic character of the metal would prob- 



