[satterly] viscosity EXPERIMENT 123 



or 1800 cms. per second, 

 which corresponds to a flow of about 35 ces. per second. Reference 

 to curve (a) in Fig. 2 will show that at the point corresponding to 

 this flow the graph changes from the straight line to the curve. In 

 this way the student is able to verify an interesting fact. 



Objections to this method of getting the viscosity of air are: 



(1) The volume of air drawn through the tube CD is not quite 

 the same as the volume of water which runs out at the nozzle N. 



(2) The mean radius of the effective portion from the tube viz., 

 CD, is got from measurements at the ends A and B. This may 

 introduce a considerable error, especially as in the calculation the 

 radius is raised to the fourth power. 



Advantages of the method are: 



(1) Simplicity. 



(2) It serves to show the student how the viscosity of air may 

 be used in the design of a meter to measure the flow of a stream of 

 air. 



From 1908 to 1910' the author used such a meter when collecting 

 the radium emanation from the air by passing an air stream through 

 coco-nut charcoal tubes where the emanation is absorbed. In that 

 instance the capillary tube was a narrow copper tube and connections 

 to the pressure gauge were made by soldered joints and india-rubber 

 tubing. A detailed study of such meters has i)een made by A. H. 

 Benton, of the Gas Mask Research Bureau of U.S.A.^, and there is 

 no doubt that gas-flow meters designed on the basis of viscosity 

 resistance will prove of great value in many experiments where the 

 flow is too small to allow of the use of any other cheap form of meter. 



Another and easier way of performing the experiment is to use 

 a gas meter instead of aspirator and graduated cylinder. It would 

 be poor teaching, however, to use an expensive gas meter in an 

 experiment designed with a view to simplicity of apparatus. In 

 some experiments with a gas meter reading to thousandths of a cubic 

 foot, measurements were taken when compressed air at a steady 

 pressure passed firstly through the gas meter and then the viscosity 

 gauge and secondly when meter and gauge were interchanged. For 

 tube B the two graphs coincided up to a pressure difference of about 

 six cms. of water and then separated, going along two practically 



^Satterly, Phil. Mag., Oct., 1908, and July, 1910. 



•■'Benton, Journal of Industrial and Engineering Chemistry, July, 1019. 



