A CENTUEY'S PROGRESS IN PHYSICS 353 



fields the deflection produced by one was just balanced by 

 that caused by the other, and from the field strengths 

 employed both the velocity of the particles and the ratio 



M 



of charge to mass was calculated. The former was 



found to be about one-tenth the velocity of light, but the 

 most startling result of the experiment was that the same 



M 



value of was obtained no matter what residual gas 



was contained in the tube or of what metal the cathode 

 was made. 



To calculate e and then m other methods are necessary. 

 C. T. R. Wilson has shown that in supersaturated air, 

 water drops form easily on charged molecules, and that 

 negative ions are more effective in causing condensation 

 than positive ones. By making use of the results of this 

 research Thomson has been able to measure the elemen- 

 tary charge. For suppose a stream of negative ions to 

 pass through supersaturated air. A little drop forms 

 on each charged particle, and the cloud of condensed 

 vapor settles to the bottom of the vessel. The charge 

 carried and the mass of water deposited can be meas- 

 ured directly. Stokes' law for the rate of fall of a 

 minute particle through a gaseous medium enables the 

 average size of the drops to be computed from the 

 observed rate of descent of the cloud. Hence the number 

 of drops formed and the charge carried by each follows 

 at once. H. A. Wilson improved the method by noting 

 the effect of an electric field upon the rate of fall of the 

 charged drops, and subsequent experiments undertaken 

 by Millikan 6 have been of such a character as to enable 

 him to follow the motion of a single drop. Instead of 

 water, the latter uses oil drops less than one ten- 

 thousandth of a centimeter in diameter. A drop, after 

 one or more electrons have attached themselves to it, 

 is actually weighed in terms of the charge on its surface 

 by applying an upward electric force just sufficient to 

 balance the force of gravity. Then its weight is inde- 

 pendently obtained from the density of the oil and the 

 radius of the drop as determined by the rate of fall when 

 the electric field is absent. Comparison of these two 

 expressions gives 4-774 ( 10 )~ 10 electrostatic units for the 



