CLARK. — ELECTRIC HEATER AND AUTOMATIC THERMOSTAT. 603 



of mercury, etc., bring its indications under suspicion. With the 

 platinum thermometer just described, it is possible to follow fluctua- 

 tions which ought to be visible in the mercury thermometer, but which 

 are not as a matter of fact. Much interesting and valuable informa- 

 tion was gained by use of this instrument. It was discovered that the 

 incandescent lamp behind the box used for illumination caused a rise 

 in temperature of over 1/10°. A lamp in the room which shone into 

 the front window affected the temperature of the thermometer no- 

 ticeably. For work on liquids near the critical point this fact must 

 not be overlooked. It is essential that the very smallest amount of 

 light possible be used, particularly when the light shines on a portion 

 only of the tube, which contains the liquid under experiment. Most 

 observers have not taken sufficient pains in this matter. Many tests 

 for constancy of temperature have been made. The following (Table 

 I) may be regarded as typical, and show the possibilities of the ap- 

 paratus. The first set (Temp. I) was obtained by breaking the entire 

 current of 3.96 amperes, the second (Temp. II) when the current 

 varied between 2.62 and 3.90 amperes. 



