CH.\FFEE. — IMPACT EXCITATION OF ELECTRIC OSCILLATIONS. 301 



sented by the very steep resistance curve for a short gap. A decrease 

 in resistance means an increased current, but the increase in current 

 further causes a decrease in resistance. This action promotes an un- 

 stable condition, and would cause a tremeadously sudden rush of 

 current if the inductance of the whole circuit did not exert a con- 

 trolling action. The effect is, however, that the terminals of the gap 

 and the wires in the immediate vicinity of the gap are suddenly robbed 

 of electricity, and because of the inductance of these terminal lead 

 wires, the terminals are charged in the opposite direction. When 

 the potential of the reversed terminals attains a sufficient value this 

 momentary current rush stops and reverses. It is evident, without 

 further consideration, that the conditions are right for the observed 

 superposed oscillations, these oscillations being more intense at the gap 

 and extending back with diminishing amplitude into the primary 

 circuit. This effect will be very apparent in some of the pictures yet 

 to be considered. 



Cut e of Plate 3 is a photograph taken when, on account of the dry- 

 ness of the hydrogen, practically nothing but a glow discharge could 

 be obtained. The gap in this particular case was traversed by a high 

 frequency current. The photograph is introduced only for the small 

 curved point at the right hand end of the potential deflection, which is 

 the characteristic of the glow discharge. 



Cut /of Plate 3 was taken in the same manner as the pictures just 

 described except for the addition of a secondary deflection coil parallel 

 to the primary coil about the tube. In this way there is obtained a 

 spreading out of the diagram, and a clearer view of the actual path 

 of the luminescent spot. The helical path from right to left results 

 from the secondary current oscillation, and superposed primary potential 

 oscillations due to the influence of the secondary vibrations on the 

 primary circuit, both occurring during the charging of the primary con- 

 denser. The abrupt end represents the point at which the gap breaks 

 down and the spot moves very rapidly, at first, over the primary 

 discharge loop. 



The change in resistance with changing current, derived from these 

 E-I characteristics, gives curves of the general shape shown by the full 

 line curves of Figure 16. The curve marked L was derived from cut a 

 of Plate 3, and represents the change in resistance of a very long gap. 

 The other two full-line curves are for shorter gaps, and, in the extreme 

 corner, is suggested the condition for decreasing current which com- 

 pletes the cyclic resistance changes. 



It is seen, on examination of Figure 16, that the dotted curve, derived 

 from an assumed expression for the primary wave form, agrees well in 



