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. Wertheim 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- 



