SOME CONTEMPORARY ADVANCES IN PIIVSICS~XI 479 



magnetic field in n direction parallel to the surface. Then we have the 

 situation which occurs in measuring the speeds and charge-to-mass 

 ratios of electrons and ions by the method of electric acceleration fol- 

 lowed by magnetic deflection. The only differences are, that in the 

 present case the speeds v with which the electrons enter into the mag- 

 netic field are imparted to them not by an imposed electric field but bv 



A9 



Fig. 7 



the radiation which released them; and that the experimenter^takes 

 the value of e/m for granted and computes the values of v from the 

 magnetic deflections alone. The electrons are swept around in circular 

 arcs, of which the radii >-ield their speeds. 



The apparatus by which such experiments are performed is of the 

 type shown in Fig. (i. At S there is a long narrow rod or tube of the 



Fig. 8 



material to be tested; it is irradiated by X-rays proceeding from a 

 source beyond the diagram to the left. A magnetic field, directed nor- 

 mally to the plane of the paper, sweeps the emerging electrons around 

 in circular arcs, some of which pass through the slit. The appearance 

 of films laid along the top of the block Pb, normal to the plane of the 

 paper, is shown b>- Figs. 8 and 9. They suggest spectra ; and though the 

 lines are signatures of special electron-speeds rather than of special 



