Fire and Heat 



89 



heit scale, reading from 92 to 110, but with each degree 

 subdivided into fifths. How does a physician take one's 

 temperature? How does he make the mercury return to the 

 bulb after using? 



A common thermometer is the aneroid or metallic, which 

 usually consists of brass and steel strips made into a spiral coil. 

 The thermograph (Fig. 26) is similarly constructed, but besides 

 indicating the temperature changes, records them on a chart 

 graduated for days and 

 hours and moved by 

 clockwork. The princi- 

 ple of the metallic ther- 

 mometer is precisely 

 the same as that of the 

 liquid or gas thermome- 

 ter. Certain metals ex- 

 pand and contract in 

 definite amounts with 

 thermal changes and 



FIG. 26. Thermograph. A seven day record 

 sheet is fastened to the cylinder which is turned 

 by clockwork. 



are utilized to show such changes. Their changes in volume 

 are accurately determined for given differences in temperature, 

 just as in the case of mercury or hydrogen. 



The ordinary mercury thermometer is useless for tempera- 

 tures above 357 C. (680 F.), for mercury boils at this tem- 

 perature. It may be made useful for temperatures as high as 

 500 C. (932 F.) by filling the space above the mercury with 

 nitrogen. Higher temperatures, however, are needed in many 

 manufacturing processes. In the electric furnace, temperatures 

 may reach 3600 C. to 4000 C. In scientific investigation 

 a temperature of 7800 C. has been obtained. (See page 175.) 

 Such temperatures are above the range of ordinary ther- 

 mometers. They may be approximated by estimating from 

 the color and the quantity of the light produced. Thus scien- 

 tists estimate the sun's temperature to be about 6000 C. 



Instruments, called pyrometers, are used for very high tern- 



