APPENDIX. 



203 



DIFFERENTIAL THERMOMETER. 



The diffcreutial thermometer consists of a o-lass tube with one end o])en 

 and the other closed with a bulb. The lower part of the tube is filled with 

 mercury and the bulb contains alcohol. As the temperature rises the alcohol 

 expands and the mercury rises in the open tube. When the alcohol con- 

 tracts the atmosphere forces the mercury into the closed tube. The instru- 

 ment is usually provided with two scales with the orders reversed as shown 

 in the figure. By having a platinum wire blown in the bend of the tube 

 and putting a wire in the open tube this thermometer can be substituted for 

 the thermometer of Fig. 1. If the instrument is to give warning of one 



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Kiff. ■_*— Differential Thenmmieter. 



Fig. !V— Thermostat. 



temperature only, and is not to be adjustable, the coil of Fig. 1 can be dis- 

 pensed with by having a platinum wire blown in the bulb of the differential 

 thermometer with the lower end of the wire opposite the required tempera- 

 ture. In this case the battery bell and differential thermometer are con- 

 nected in series, i. e., so that the current floWvS when the mercury touches 

 the wire blown in the bulb of the thermometer. 



THERMOSTAT. 



Fig. 3 represents one form of the thermostat, of which there is a great 

 variety. It consists of brass and steel rods fastened together. The brass 

 expands or contracts more with a given change of temperature than the 

 steel and thus the rod bends as the temperature changes. In the figure I is 

 the brass rod and B the steel rod. With a lowering of the temperature the 

 rod will bend toward the right until it touches the screw S, which is set at 

 the required temperature. It is connected in series with the battery and 

 bell. A very slight derangement will make the thermostat useless, and. for 

 practical uses and accurate results, it is not so desirable as the thermometer. 



The writer would be pleased to correspond with any one wishing further 

 information on this subject. 



