DIFFERENTIAL AIR THERMOMETERS. 



271 



125 represents a more neatly finished form of differential air 

 thermometer where the two bulbs and connecting stem are blown 

 in one piece. 



A better instrument results if the corks are doubly perforated 

 and the second perforations supplied with short 

 bits of glass tubing about an inch long, the two 

 being connected by a piece of thin india-rubber 

 tubing; there is thus a direct communication 

 between the two bulbs through the india-rubber 

 tube, so that any inequality of level that might 

 otherwise accidentally exist in the liquid in the 

 U tube is obviated by the equalisation of pressure 

 thus brought about. When the thermometer is 

 to be used, the india-rubber tube is closed by 

 means of a spring pinch cock or screw clamp 

 (fig. 126), so that the direct communication be- 

 tween the bulbs is cut off. A further improve- 

 ment is to use a longer glass tube and bend over 

 the ends again at right 

 angles, so that the flask 

 bulbs may depend down- 

 wards, and so be capable 



of being P laced in dif - 



eeter. <f ent ssels f w ^f*> Fig . 126 . Screw Pinch Cock . 



&c. When the bulbs 



and tube are blown in one piece the connecting rubber tube and 

 clamp are replaced by a cross piece of glass tubing provided with a 

 tap, so that when the tap is open the two pendent bulbs are in direct 

 communication, and when closed the thermometer is ready for 

 use; this form is known as " Matthiessen's differential air- 

 thermometer," the one with upright bulbs being " Leslie's " form 

 of the instrument. 



CHAPTER XX. 

 CONVECTION AND CONDUCTION OF HEAT. 



Expt. 318 shows that when a gas is heated under such circum- 

 stances that it can expand and move about freely, it becomes 

 lighter and ascends bodily, thus setting up currents. This process 



