PHYSICS, CHEMISTRY AND ENGINEERING 187 



under static load as given by engineers' hand books or as de- 

 termined from test results. If the use of equation (2) or 

 equation (3) with a proper factor of safety gives higher re- 

 sults than the safe static stress, it means that static stress con- 

 ditions are the determining conditions, and that there is more 

 danger of failure by static stress than by fatigue. 



TABLE OF VALUES OF THE CONSTANT B. 



Material B 



Structural Steel and Soft Machinery Steel 250,000 



Wrought Iron 250,000 



Steel, 0.45 per cent Carbon 350,000 



Cold-rolled steel Shafting 400,000 



Tempered Spring Steel 400,000 to 



800,000 



THE DETERMINATION OF V AND E/M FOR 



CATHODE RAYS AS A LABORATORY 



EXPERIMENT IN PHYSICS 



Chas. T. Knipp, University of Illinois 



The writer has endeavored during the past two years to de- 

 vise a method and apparatus 1 for the determination of v and 

 e/m for cathode rays with sufficient accuracy to make it a de- 

 sirable experiment for undergraduates in electrical measure- 

 ments. 



The parallel field method of J. J. Thomson has been simpli- 

 fied by using slow velocity cathode rays, such as are obtained 

 from a Wehnelt or hot lime cathode. 2 These slow moving 

 rays make it possible to use the earth's field for the magnetic 

 deflection in place of the ordinary electro-magnet. This sim- 

 plifies the formula and does away with the end corrections due 

 to the spreading field at the edge of the magnet. The elec- 

 trostatic deflection of the rays is obtained by the usual method 

 of placing two parallel plates within the discharge tube. 



With a beam 30 cm. long the magnetic deflections are of the 

 order of one centimeter. This may be increased or dimin- 

 ished by using a lozu or a high potential in operating the Weh- 



'Knipp, School Science and Mathematics, Vol. 14, 1914. 



3 Knipp and Welo, Terrestrial Magnetism and Atmospheric Electricity, June, 



