CLARK. — ELECTRIC HEATER AND AUTOMATIC THERMOSTAT. 601 



When the proper amount of current is used the re^'ul.ation near 200^ 

 is witliin 1 , 10° when the entire current is cutolF. The adjustment fi)r 

 this accuracy nee<l not l)e very carefully made. When a portion only 

 of the current is cut off and the adjustment be made with sufficient 

 care the variation in temperature may be made very small. Close 

 regulation at low temperature is much easier than at higher and there 

 is less need of careful adjustment; as the temperature is carried 

 higher regulation becomes more difficult. One source of difficulty in 

 nu\intaining constancy of temperature is due to the fact that heat is 

 conducted along the mercury in the steel tube connecting the mercury 

 chamlier with the reservoir attended by a rise of temperature of the 

 mercury in the reservoir. This rise in temperature has been obviated 

 bv surrounding the reservoir with a coil of small lead tubing (shown in 

 Fig. 1) through which a current of cold water is kept circulating. 



The table shows the values of the currents necessary to maintain the 

 heater at different temperatures: — 



Amps. Temp. 



1.43-1.55 67.2 



2.35-2.45 100.6 



2.88-3.05 198.0 



4.30^.40 221.0 



At 198° with the regulator changing the current from 2.88-3.05 a 

 thermometer graduated to 1 /5° was watched through a microscope and 

 no motion of the thread was apparent. The regulator worked at 

 about two-minute intervals. One very serious difficulty which gave 

 trouble for a long time was with the lubrication of the bearings of the 

 shafts, <lri\ing the fans in the inner box. Ordinary lubricating oils 

 boil out of the bearings at about 180° and condense on the windows 

 of the outer box, obscuring the \'iow of the inside of the box. Below 

 about 180° there is no trouble but above this the distillation of oil 

 occurs. Various oils were tried with no success because there is always 

 this distillation at some temperature. Finally the difficulty was over- 

 come by using paraffin wax which melts at about 50° and does not 

 distil away enough to cause any trouble. Small pieces are placed 

 in the ends of the oil tubes leading to the inner bearings. These are 

 quickly melted by the heat conducted from within and run down to 

 the bearings lubricating them very efficiently. The slight jarring of 

 the whole apparatus causes trembling of the mercury at the relay 

 contact and no sticking of the mercury to the platinum point has lieen 

 noticed. A little alcohol on top of the mercury helps to keep it clean. 

 Not only is it important that there shall be no unsteadiness of tem- 



