RESPONSE BY VARIATION OF ELECTRICAL RESISTIVITY 545 



any complication such as might result from the passage of a 

 strong current. 



In order to subject the specimen to a gradual and 

 continuous rise of temperature, it was placed in the thermal 

 chamber, which has already been described (fig. 131). Before 

 the gradual raising of the temperature is initiated, an exact 

 balance is first obtained, the galvanometer spot of light being 

 thus adjusted to zero. This position can be maintained for an 

 indefinite length of time, provided the specimen is subjected 

 to no variation of temperature. We have already seen that no 

 resultant electro-motive variation is induced, in consequence 

 of stimulus, in a physiologically isotropic tissue. Any 

 change now recorded under a gradual rise of temperature, 

 by the movement of the galvanometer spot of light, must, 

 therefore, be due to a resulting variation of resistance. The 

 movement of the galvanometer spot of light is recorded in 

 the usual manner, on a photographic plate, a down-record 

 representing an increase of resistance, and an up- record a 

 diminution. In order that the curve should also give indica- 

 tions of different temperatures, light is cut off for a short 

 time at every 2 C. of rise of temperature. Thus each of the 

 successive gaps in the record indicates a temperature-ascent 

 of 2 C. 



Taking now the specimen of pistil of Hibiscus , balanced as 

 described, it was seen, on beginning gradually to raise the 

 temperature, that the balance was upset, while the growing 

 deflection of the galvanometer spot indicated an increasing 

 resistance. The method of experiment, which has been 

 described, proved now so delicate that it was impossible to 

 record the entire curve within the range of the photographic 

 plate. It was, therefore, necessary to choose for record only 

 that part of the deflection which included the interesting and 

 significant point of inversion. The photographic record 

 thus commences at 56 C, it being understood that there 

 has been, before this, a larger and continuously growing 

 deflection downwards, indicative of increasing resistance. 

 During record the deflection continues to increase, till the 



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