Nipher— Nature of the Electric Discharge. 15 



To take a rough illustration, a column of sand and a 

 column of shot having equal cross-sections, might have 

 their lengths so adjusted that they would offer equal 

 resistance to the flow of an air current. The linear veloc- 

 ity of air molecules would be very different in the two 

 columns, if one were to replace the other in the same cir- 

 cuit. A column of cotton fiber might have the same spe- 

 cific resistance as a column of shot, and the linear veloc- 

 ity of the air molecules might be very different when the 

 same current was being forced through them. 



It seems possible that abrupt changes in velocity of the 

 corpuscular current in a circuit where unlike metals are 

 involved may be an important element in the thermal 

 behavior of such circuits, at points where unlike metals join. 



The sudden change in velocity of corpuscles when they 

 leave the luminous conducting channel of an electric arc 

 and enter the positive carbon, is an illustration of the 

 action here discussed. Such a change would account for 

 the high temperature of the positive carbon. 



An iron wire forming part of a circuit the rest of which 

 is copper, and through which a current flows, is heated 

 at one junction and cooled at the other. These effects 

 are exchanged on reversal of the current. The resulting 

 temperature will depend upon specifie heats of the two ma- 

 terials, as well as on radiating surface per unit of mass 

 in which the heat is developed. All of the properties in- 

 volved vary with temperature, and some of them are very 

 imperfectly understood. 



It has long been known that when metals are heated 

 they exhibit electrical properties. Through a certain 

 range of temperature a metal will ''emit positive elec- 

 tricity,'^ or as we would now say, it will drain negative 

 electricity from the surrounding air. This effect is great- 

 est at a certain temperature, which is different for dif- 

 ferent metals. 



The positive ions which are at the same time urged 

 from the heated metal are molecules of air or other gas or 

 of the metal terminal, which have been drained of their 



