255 
1909-10.] On an Electrically Controlled Thermostat. 
collar which prevents upward displacement. The adjustable driving pulley, 
not shown in fig. 1, is fixed a little lower down. If the full degree of 
constancy is to be attained, rapid stirring, low capacity for heat of the heating- 
apparatus, and high sensibility of the thermo-regulator are essential. For 
most purposes a constancy of about one-hundredth of a degree centigrade 
is amply sufficient, and the rate of stirring may be correspondingly reduced. 
The heating apparatus consists of electric glow-lamps with elongated 
glass necks placed in the water of the bath as shown in fig. 1. These 
lamps were made by the Ediswan Company, according to Professor Hugh 
Marshall’s specifications. 
Glow-lamps with water-tight metal covers to protect the insulation of 
the wires were also tried, but they were not satisfactory. The air inside 
the cover and the glass of the glow-lamp acted as a store of heat, which 
continued to raise the temperature of the bath after the lamp had been 
extinguished. This caused a “ lag ” of several hundredths of a degree. 
The thermo-regulator finally adopted consisted of a long spiral glass 
tube filled with toluol, and terminating in the U-shaped reservoir e (figs. 1 
and 2) containing mercury. 
