196 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1924 



The direct current of such generating machines was much later 

 followed by the alternated flow, and soon alternating-current gen- 

 erators were made for different rates of reversal of direction. Sixty- 

 cycle and twenty-five- cycle currents are now common, and the user 

 takes his choice. These frequencies were once accidents of conven- 

 ience and economy. For some uses other widely different frequencies 

 of alternation are very desirable, as in radio, where 1,000,000 cycles 

 are common. 



Now the phenomena which have been found in vacuum tubes prom- 

 ise to give complete control over all these details of kind and fre- 

 quency of current. 



ELECTRICAL CONTROL 



As will be more fully shown later, when a unidirectional current 

 meets vacuum tubes as though it would pass through them, it must 

 find one particular kind of a tube, and the current's direction must 

 be right, because some tubes will let it through only when it is both 

 unidirectional and in the proper direction. These in themselves are 

 rather remarkable things to expect of a vacuum, but as usual the 

 truth exceeds the expectation. 



CATALYSIS 



To give some idea of the extent to which vacuum studies may 

 affect remote fields, mention may be made of chemical catalysis, the 

 secret of most reactions of life. For example, it is known that 

 mercury vapor in a vacuum, w^hen illumined by light of a certain 

 wave length, will absorb that light and turn the energy over to 

 hydrogen, if hydrogen be present, so that this, in turn, will chem- 

 ically reduce such substances as cold copper oxide. Here is a new 

 kind of chemical process. It is the kind we have needed in order to 

 begin to explain some of those life reactions which vegetation dis- 

 closes. That is, similar facts will probably be found to contain the 

 explanation of the catalytic action of sunlight on growing plants. 



And so the studies of phenomena in vacua may lead us into the 

 most widely separated fields. The experiments described in the lat- 

 ter part of this article are illustrations of this fact and are thus not 

 chosen to be very closely related. 



KINDS OF VACUA 



One might say, as in our school-day essays, " There are different 

 kinds of vacvumi, too numerous to mention," and then proceed to 

 mention them all. However, only a feAv cases will be selected for 

 the purpose of illustration. A certain kind of vacuum is good 

 enough for incandescent lamps, because other factors besides the 



