1004 Drs. Foote, Meggers, and Mohler on Excitation of 



to the applied potential. These electrons then collide with 

 atoms in the large force-free space between the plate and 

 grid, while the impacted atoms, in approaching equilibrium 

 condition following collision, emit radiation which may be 

 photographed. 



Fig. 1. 



V/4R/ABLE HELD 



Arrangement of electrodes. 

 The grid must be mounted very close to the hot wire. 



With an arc of this type in which electrons of any desired 

 definite velocity may be secured, one may expect to obtain 

 results of some interest in the theory of atomic structure. 

 On the other hand, in an ordinary capacity discbarge or arc, 

 where the potential is applied across the discharge-gap, we 

 know nothing of the actual velocity attained by an electron 

 at the instant of a collision with an atom. The velocity 

 developed per mean free path probably determines the 

 general characteristics of the arc, but even when this mean 

 velocity is low some electrons may be accelerated, without 

 collision, for many free paths, thus accumulating a high 

 velocity. We accordingly find in the ordinary arc many 



