CHAFFEE. — IMPACT EXCITATION OF ELECTRIC OSCILLATIONS. 309 



proof of the existence of linear damping in a circuit containing a spark 

 gap, and to show that there are two distinct forms of the oscillation 

 trains according to the nature of the gap. 



(7) Xotes Concerning the Mechanism of Conduction in the Cu-Al Gap. 



Very little can at present be given concerning the exact nature of 

 the conduction which takes place in the gap under discussion and the 

 office of the necessary hydrogen. No oscillations can be obtained un- 

 less the gap is immersed in some gas rich in hydrogen, but the latter 

 alone gives by far the best results. In any case the gas must be moist. 

 As was shown early in Part I, the absence of metallic vapor indicates 

 that the conduction is probably affected entirely by the ionization of 

 the surrounding gas. Since, as was also observed, the metal used for 

 the anode makes little difference in the operation of the gap, and since 

 aluminum is the only metal which works at all well as cathode, it is 

 safe to say that it is the aluminum cathode, working in conjunction 

 with the hydrogen, which gives the characteristic properties of the 

 oscillation gap which has been partially studied in the foregoing 

 pages. It might be mentioned here that magnesium acts to some ex- 

 tent as cathode, but, on account of its low vaporizing point, works very 

 irregularly and unsatisfactorily. The discharge, in this case, is in- 

 tensely green showing vaporization. 



The fact that moisture in the hydrogen is necessary indicates that 

 an oxidation of the aluminum plays some part in the operation. The 

 necessity of an hydrogen atmosphere leads one to believe that perhaps 

 a reduction of the ever present aluminum oxide film at some time 

 during the cycle may take place, there pc ^sibly being a cyclic change 

 of oxidation and reduction of the aluminum determined by the tempera- 

 ture at various instants during one primary discharge. For instance, 

 it may be that some action such as follows takes place. At the begin- 

 ning of the discharge, the thin oxide film makes necessary a high 

 breaking-down potential in order to start a discharge. As soon, how- 

 ever, as the discharge is started, H ions are conveyed to the aluminum 

 cathode, and, since the atmosphere about the terminal is H, reduce the 

 oxide film. The free surface of the aluminum is thereby exposed, and 

 the resistance drops to a low value. This reduction would be the 

 greater the larger the current, and there would result the very rapid 

 drop in resistance which is shown in the curves of Figure 16. As soon 

 as the discharge stops the aluminum partially reoxidizes, thereby 

 reestablishing the initial high resistance. 



It is probable that the regularity of the gap is partly due to the fact 



