:;:'>! Kr aider— Eaiperiments in Elementary Physics. 



indicates the pressure under which this known volume of gas 

 exists; and there is no mystery as to how the gas got there or 

 how the equilibrium condition was ever practically attained, 

 which were two unprofitable difficulties presented to the 

 thoughtful student by the older apparatus. The class is then 

 asked to calculate the position of the mercury surface in the 

 reservoir when it is so moved as to reduce the volume of gas 



to (say) 3. They assume that V cc p ; and since Y 2 = £ V, 



P 3 = $-P, = i in excess of the barometer = 10" 4- Bar. The 

 adjustment is then made and the zero of the tape is moved 

 into alignment with the nearest mercury surface, which is 

 seen to be 10 inches from the other. After one such demon- 

 stration most of the class will promptly answer for the cases 

 when Vj= 5, 2, 6 or 8, in all of which the numerical work is 

 so simple, when the barometer is 30 inches, as to be easily and 

 promptly handled mentally. For the purposes of a lecture 

 demonstration a barometer of 30 inches may be assumed with- 

 out trouble in the experiment, except on the rare occasions 

 when the atmospheric pressure departs greatly from the normal. 

 It has the great advantage of simplifying the numerical work 

 and, when followed by problems which are sufficiently varied, 

 no harm is done. While the class is making its mental calcu- 

 lation the desired adjustments may be made, without preju- 

 dicing the results, providing the tape is meanwhile kept in motion 

 or turned with its printed side back. The experiment can of 

 course be varied by starting with any volume and pressure and 

 with a fixed scale, as with any other apparatus. 



Sloane Physical Laboratory, 

 Yale University, 

 Aug. 10, 1911. 



