HEAT EXCITES ELECTRIC CURRENTS. Ill 



and capable of readily turning around it. A cir- 

 cuit is formed by a cross-piece, w E, made of some 

 less freely conducting metal than the arching 

 piece of wire, for the purpose of determining the 

 direction of the circulating thermo-electric current 

 excited by the heat of the flame applied at the 

 joint w. Were equally conducting metals used, 

 the vibratory excitation of heat would be extended 

 by molecular vibration of the wire (denoted " con- 

 duction of heat"), instead of circulating electric 

 currents. The excitation from the flame of the 

 lamp first ascends the readily conducting side of 

 the arched frame, and descends on the other side, 

 completing the circuit through the less readily 

 conducting metal. The ascending current from 

 w is vertical, and at right angles to the horizontal 

 terrestrial currents ; and produces a movement of 

 the frame at right angles to the plane of the 

 frame (as indicated by the direction of the small 

 arrow on the conducting-wire No. 7, Fig. 29). 

 The vertical descending current on the other 

 side of the frame corresponds with that of the 

 conducting-wire No. 3 ; and produces a move- 

 ment in an opposite lateral direction, as denoted 

 by the small arrow on c D. Both combine to 

 turn the frame horizontally on its pivot. When 

 the side E comes over the flame, the heat reverses 

 the current ; and these alternate reversals keep the 

 frame continually turning. A similar revolving 

 motion of a little frame is described by Professor 



