soArE coNrEMi'OR.iRy .iDf.ixcr.s IX I'liysics-ui 



287 



The art of tletcctinR radiations by this inothod consists in givinn 

 various values to the "Ifomharding \-oltagi'" V l)clwecn target and 

 fdanicnt, which is the measure of the energ;y of the electrons im|)inging ■ 

 on the target; nieasurinR the photoelectric current /, which is the 

 measure of the intensity of the rays; plotting / (or better the ratio of « 

 to the current of bombarding electrons) \ersus V; and examining the 

 cur\e to see whether it displays sudden changes of slope. If it does, 



Fig. 8 — Mohler and Foote's apparatus tur <litcriiiining excitation-potential: 

 gases and vapours. (Bulletin of the Bureau of Standards.) 



one infers that at the corresponding voltages new radiations suddenly 

 burst forth. The method therefore consists in finding critical bom- 

 barding-voltages, that is, critical electron energies which just suffice 

 to excite particular sorts of radiation; it is a method for discovering 

 excitation-potentials. Three excellent instances of such abrupt changes 

 in slope, or breaks as they are frequently called, appear in the (i, V) 

 curve determined with an aluminium target by Horton and his as- 

 sociates (Fig. 9). Very many such curves appear in the literature, 

 with more or less conspicuous breaks; some are as striking as these 

 in the figures, some require a good deal of care and experience to locate 

 them properly, and some, one is driven to conclude, are visible only 

 to the eye of faith. But it is hardly possible to doubt that such a 



