Magnus on Thermo-electric Currents. 



85 



current, although the radiative powers of both portions must 

 have been very different. It is thus proved that a difference in 

 respect to radiative power is not sufficient to originate a thermo- 

 current. 



It has been already stated, that where a difference in point of 

 hardness exists, a current is produced. To examine this point 

 further, a number of wires each 6 feet long and 045 of a line in 

 thickness were chosen ; and of those which could bear a high 

 temperature, two feet in the middle were heated to redness and 

 thus rendered soft. Of the more fusible metals, such as tin, 

 lead, zinc, &c, two feet were heated in an oil-bath at 200° C. 

 for an hour. "When cooled, the two ends of each wire were 

 united with the galvanometer ; one of the junctions between hard 

 and soft was heated, and the consequent current observed. The 

 following table exhibits the results of these experiments : it will 

 be observed that the direction of the current does not preserve a 

 constant relation to hard and soft. In some cases it flows from 

 soft to hard, in other cases in the opposite direction. 



By means of the pretty little instrument represented in fig. 4*, 

 and which its inventor has named the monothcrmic pile, the 

 action may be considerably increased. A length of hard brass 

 wire is taken, every alternate six inches of which are rendered 

 soft by heating to redness. Thus six inches of soft wire succeed 

 six inches of hard throughout the entire length. The wire is 

 then wound round a frame of suitable size, and presents when 

 wound the appearance of a rectangle, two of the opposite sides 

 of which arc composed of hard and soft wire respectively; the 

 centres of the other two sides are the junctions of hard and soft. 



* Another form of this instrument is represented in fig. 5. 



