246 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



The collisions of electrons and ions with atoms are probably not 

 physical contacts as when a baseball bat hits a ball, but rather the 

 repulsion between the charges on the individual electrons and ions. 

 The molecules of a gas are in a constant state of violent motion and the 

 electrons, in turn, are moving about in their respective orbits. It can 

 readily happen that the relative directions of motion of the electrons 

 are such that an electron is caused to leave its parent atom and 

 produce ions. 



An interesting byproduct of ionization occurs when a colliding 

 electron does not have sufficient velocity to produce ionization but 

 does cause an electron in an outer orbit to jump to another orbit 

 of higher energy level. The energy absorbed by this change of orbit 

 is released in the form of a visible electromagnetic wave as soon as the 

 electron drops back into its original orbit. The visible radiation or 

 light is of a color characteristic of the element constituting the gas 

 molecule. Frequently the light is monochromatic. This principle 

 of light production underlies all of the gaseous and vapor electric 

 illuminating sources, such as the neon, mercury-vapor, and sodium- 

 vapor lamps. 



RESISTANCE TO CURRENT FLOW 



Electric resistance is that property of a circuit which opposes the 

 passage of a current. It is easily explained on the basis of the electron 

 theory. In a solid conductor resistance is due to the collision of the 

 electrons which constitute the current with the atoms and molecules 

 of that solid. The magnitude of the resistance depends on the num- 

 ber of free electrons available. In a similar manner, the resistance of 

 an electrolyte or gas is determined by the number of ions produced 

 and by the number of collisions between the moving ions and the 

 molecules present. The influence of temperature on resistance is 

 readily explained. In a pure metal a rise in temperature increases 

 the kinetic thermal energy, speeds up the motion of the molecules, 

 and hence tends to increase the number and the violence of the 

 collisions, thus increasing the resistance. 



It is of interest to note that the "hither and yon" movement of free 

 electrons in solids does produce infinitesimal differences of potentials 

 or voltages at the ends of conductors. These minute voltages intro- 

 duce noise termed "resistance noise" in high-gain amplifiers and limit 

 the smallness of signals which can be amplified satisfactorily. 



ELECTRONIC DEVICES 



The first electronic device was the two-electrode tube patented by 

 Fleming in 1903 and known for several years as the Fleming valve. 

 It took the place of the coherer as a detector of wireless telegraph 

 signals. This device has grown to be the detector and the rectifier 



