' Atoms in Electric Discharges. 481 



August number of the Philosophical Magazine (1890), "On 

 the Velocity of the Transmission of Electric Disturbances," 

 which contains the following passage : — 



" The very rapid rate with which the electric discharge is 

 propagated through a rare gas compels us to admit that the 

 electricity is not carried by charged atoms moving with this 

 velocity. For if it were, then if the discharge were to take 

 place in air at atmospheric pressure between two parallel 

 plates one centimetre apart, charged to a potential difference 

 of approximately 30,000 volts, the kinetic energy which would 

 have to be communicated to the atoms to make them move 

 with this velocity would be greater than the original potential 

 energy of the charged plates, assuming that the charge on 

 each atom is that deduced from electrolytic considerations." 



The unusual dispersion afforded by a Rowland concave 

 grating led me to test this hypothesis in as far as it relates to 

 the question, Are the molecules of metals carried with the 

 oscillations of electricity from terminal to terminal between 

 which the oscillations take place ? 



A circuit of wire giving a suitable value of self-induction 

 was arranged in connexion with a series of Ley den jars. The 

 time of oscillation was calculated from the well-known for- 

 mula t = 27r\/hC, in which L is the value of the self-induction 

 of the circuit, C the capacity of the Leyden jars. Preliminary 

 examination of the electric spark taken through this circuit 

 with a revolving mirror showed that the discharge was an 

 oscillatory one. Two different values of self-induction were 

 employed. One gave the duration of a double oscillation 

 *= -0000003 of a second, the other gave £ = -0000024 of a 

 second. 



If we denote by V the velocity of light, — X and A, wave- 

 lengths, S the speed of approach of the atom, we shall have 



X= i ^ — rz |, The distance across which the oscillations took 



~ (v+s)' 



place is 6 millimetres. Calculation shows that if the iron 

 atoms were conveyed to and fro between the terminals, a 

 broadening of the iron hues in the spectrum would result 

 which could be readily detected. The broadening might 

 amount to a space equivalent to a whole tenth-metre. 



The oscillating spark passed between two iron terminals. 

 One of these terminals was hollow. The hollow terminal was 

 placed in a line perpendicular to the slit of the spectroscope, 

 so that the oscillation of the spark should be toward and away 

 from the slit. If therefore the iron atoms moved to and fro 

 with the oscillations of electricity across the air-gap, a dis- 



