THE NATURAL CURRENT AND ITS VARIATIONS 119 



allowed to return to the temperature of the room, this 

 diminution of current is annulled. 



To study the effect, on the other hand, of a rising tempera- 

 ture, the chamber is gradually heated, by means of the 

 electric heating-coil already described. In thus raising the 

 temperature, it is found (fig. 84, right hand curve) that the 

 natural resting-current undergoes an increase. On cooling 

 again to the original 

 temperature, this in- 

 crease is annulled. 

 That these effects are 

 due to induced elec- 

 tro-motive variations, 

 and not to any 

 changes of resistance, 

 is demonstrated from 

 the fact that the effect 

 described takes place 

 even when the original 

 E. M. F. is exactly 

 balanced by a poten- 

 tiometer. 



It was specially 

 stated that these ob- 

 servations with regard 

 to the effect of varying 

 temperatures apply 



FIG. 84. Effect ol Variation of Temperature 

 on Natural Current, |, which in Petiole of 

 Musa flows from Convex to Concave Side 



Effect of cooling from 30 C. to 27 C., seen 

 on left, induces negative variation of natural 

 current. Restoration to original value on 

 return to surrounding temperature. Warm- 

 ing induces positive variation (see record to 

 right). In this and subsequent figures in 



1 I 1 . ,1 J '. -A* 



the present chapter ^ indicates the direction 

 of the natural current of rest. 



only to steady varia- 

 tions. In the case of 

 thermal ascent, we 



have seen that a steady rise brings about an increase of 

 the existing current. But since sudden variation of tempera- 

 ture acts as a stimulus, we shall, in the preliminary stage, 

 obtain an excitatory reaction, which will cause a transient 

 diminution of the current of rest. This will be followed, 

 when the rise of temperature is steady, by an increase of 

 the current of rest. I give here (fig. 85) a photographic 



