RELATION BETWEEN TEMPERATURE AND GROWTH 443 



can be subjected to gradual variation. The electrolytic 

 rheostat consists of two semicircular troughs (fig. 181). By 

 turning the handle, say to the right, the electrolytic resistance 

 interposed is continuously increased, diminishing the current, 

 and hence diminishing the temperature inside the chamber. 

 Rotation of the handle in the opposite direction produces the 

 opposite effect, that is to say, raises the temperature inside 

 the chamber. Thus, by proper manipulation of the handle of 

 the rheostat, the chamber can be raised to any temperature, 



Fig. 181. Semicircular Electrolytic Rheostat inteiposed in Heating Coil 



Current enters the first trough, filled with zinc sulphate solution, by the 

 electrode, z', and is led to the second trough by the diagonal metallic 

 connector, D d'. By turning the index-arm, 1, clockwise, the interposed 

 resistance is increased, and the heating current thus diminished. Rota- 

 tion in the opposite direction diminishes the resistance and increases 

 the heatinc current. 



which can then be maintained uniform for any length of time, 

 by keeping the rheostatic resistance constant. From a 

 previous experiment, the temperature-values of the position 

 of the index in connection with the handle can be ascertained 

 and marked. Thus by turning the handle to any given index 

 number, say of 31 C, the temperature of the chamber will be 

 found to be raised permanently to that value. A delicate 

 thermometer graduated in twentieths of degrees is placed in 

 the chamber, and affords an independent indication of the 



