Thermo-electric Couples 249 



Alternate hard and soft places, the least oxidation of surfaces, 

 strain, twisting, bending, shear; all contribute to variations, 

 some to a great degree. 



The cause of thermo currents. Until the advent of the electron 

 theory, the phenomena of thermo-electricity were without ex- 

 planation, since it was definitely known that there is no exchange 

 of atoms between the metals. But the electron theory gives a 

 very good explanation of the observed facts. 



Let us consider an electron; the best assumptions of J. J. 

 Thomson attribute it to be ttoo of the mass of a hydrogen atom, 

 while the repelling force exercised between two electrons is 10^' 

 times greater than the gravitational pull exercised by the same 

 masses. Each electron is a negative charge of electricity, and 

 we consider that when a univalent molecule has two electrons 

 attached, it carries a negative charge ; when one, a neutral charge ; 

 when none, a positive charge. 



The modern view of heat is that it is a rapid motion of the 

 smallest parts of matter, that is, of these electrons, and it has been 

 determined that to raise the temperature of a wire from 0° to 

 100°, increases the velocity of its electrons 17 per cent. 



Just as the pressure of a gas is increased by heating, the ''pres- 

 sure" of the electrons is increased by heating, and they travel 

 into cooler parts, where the ''pressure" is less. 



When a wire is bent and joined, forming a circuit, and a point 

 of this wire is heated, the electrons move in opposite direc- 

 tions from the hot point; but no current results, since they go in 

 both directions. If, however, the wire on one side of the hot 

 point is cooled, the slope of pressure will be abrupt on that side, 

 and the electrons will pass in that direction. When a thermo- 

 junction is made, the "slope of the heat curve" is altered by join- 

 ing two metals of different conductivities for heat. If bars of 

 lead and zinc are joined, and the union is heated, this tempera- 

 ture slope is more abrupt on the lead side, so the electrons will 

 pass from zinc to lead. Since these electrons are negative charges, 

 the current of positive electricity is said to pass from lead to 

 zinc. 



On the other hand, if a current is sent across a cold junction, 

 the electrons passing from zinc to lead, since there is more resist- 

 ance in the lead than in the zinc, the electrons must acquire 

 energy from somewhere, which they do from their motion, hencei 



