MOLECULAR MAGNITUDES 287 



Modern methods which are used for measuring 

 small gaseous pressures also illustrate the reality and 

 the fundamental facts of molecular kinetics. A typical 

 pressure gauge consists of a chamber which is connected 

 to the vessel containing gas at the unknown pressure. 

 It contains a small strip of platinum and a movable 

 vane. The strip is heated electrically. The de- 

 flection of the vane is manifested by the rotation of 

 a beam of light reflected from it. The vane turns as 

 a result of its bombardment by molecules which have 

 been heated by contact with the platinum strip. The 

 pressure at the vane is therefore higher than elsewhere 

 in the vessel. The uniform pressure in the rest of the 

 vessel may be expressed in terms of the pressure on the 

 vane and the temperatures of the vane and the plati- 

 num strip. With a gauge of this type pressures have 

 been observed as small as 5X10~ 10 cm. of Hg (i.e. 7 

 millionths of a dyne per sq. cm.). 



