MIRACLES OF SCIENCE 



lets of water in effect miniature dew drops upon 

 the electrons. The particles of water thus formed 

 constitute a fog which begins to settle toward the 

 bottom of the receptacle. The rate at which the 

 particles settle can be determined by direct observa- 

 tion of the upper surface of the fog. 



Heavy substances, as everybody knows, fall under 

 influence of gravity at a fixed rate, regardless of size 

 or weight. But minute particles, on the other hand, 

 make their way downward through the atmosphere 

 at a rate that varies with their size in accordance with 

 a law named Stokes* law after its discoverer. This 

 direct observation of the rate of settling of the par- 

 ticles of water condensed on the electron gives the 

 size of the particles. Another computation shows 

 the total amount of condensed vapor, so a simple 

 division gives us the number of the droplets and 

 hence of the electrons. 



Then the total charge of electricity carried by all 

 the electrons can readily be measured with the elec- 

 trometer, and again all that is necessary is to divide 

 this quantity by the number of electrons to find the 

 quantity of electricity which each conveys. 



The experiment shows that the unit charge of 

 electricity carried by the electron is always the same. 

 Professor R. A. Millikan, of the University of Chi- 

 cago, has recently confirmed this by a series of in- 

 genious experiments, in which he isolates a droplet 

 of oil and observes it through a miniature telescope 

 while it takes up one or more electrons from the air. 

 His experiments permit him not only to measure 

 accurately the electric charge of an electron, but to 



128 



