﻿454 Sir J. J. Thomson on Ionization 



In order that a positively charged particle may ionize a 

 gas at all T' must be greater than W/4 ; thus if Ave lake W 

 to be represented by 10 volts, T' must be at least 2"5 volts. 

 Thus T, if the positively charged particle were an atom of 

 hydrogen, must be 2*5 x 1700 or 4250 volts; if it were a 

 molecule o£ hydrogen it would be 8500 volts, if an atom of 

 oxygen 68000 volts. Thus in the case of the positive rays 

 produced in a vacuum-tube, unless the potential difference 

 between the terminals is very large, it will only be the atoms 

 and molecules of the lighter gas which can produce ionization 

 in ihe way we are considering. 



According to this view it is the velocity of the positive 

 particle and not its energy which mnst exceed a critical 

 value before it can ionize a gas, and a knowledge of this 

 critical value would enable us to find the value of W. It 

 would I think be easier to determine W by experiments on 

 positive particles than with negative ones ; and I hope by 

 isolating by means of magnetic and electric deflexions posi- 

 tive rays of definite character and velocity, to get information 

 on this point. 



In the preceding investigations we have supposed that the 

 corpuscles in the atom were free; the amount of energy com- 

 municated to a free corpuscle will not, however, differ appre- 

 ciably from that given to one held bonnd by the forces 

 exerted on it by its neighbour, if the time of oscillation of the 

 corpuscle under these forces is large in comparison with the 

 time during which the forces due to the moving particle are 

 appreciable. This time will be of the order djY, and when 

 the particles are moving with a velocity of 10 9 cm. /sec. this 

 time will be of the order 10 ~ 18 sec, which is very small in 

 comparison with the time of any vibrations revealed by the 

 spectroscope. For very slow cathode rays or positively 

 charged particles, the influence of the forces inside the atom 

 might be appreciable and reduce the energy communicated 

 to the corpuscle. These forces will in any case naturally 

 deflect the corpuscle on its way out of the atom, and make it 

 emerge in a different direction from that in which it started 

 after its encounter with the moving particle. We can easily 

 show that this direction makes an angle 7r/2 — with the 

 original direction of motion of the particle, so that when 6 

 is small this direction is at right angles to the direction of 

 the primary rays. 



Radiation produced by the Reconciliation of Ions. 



When a corpuscle is liberated from an atom or molecule, 

 efinite amount of work W is required, which is equal to 



cl c 



