306 TR-E REALITIES OF MODERN SCIENCE 



was obtained. The numeric representing it is the 

 number of units of quantity per gram. It was found 

 to be about 1.7X10 7 when the quantity was measured 

 in electromagnetic units. 



Upon the assumption that the ion of hydrogen in 

 electrolysis is essentially the mass ra' of a hydrogen 

 atom and represents an equal (but opposite) charge 

 of electricity, the value of m was obtained. In elec- 

 trolysis one e.m. unit of quantity is transferred by 

 (1.008)(0.01118)/(107.88) or about 10~ 4 gram of hydro- 

 gen. Hence e/m' is 10 4 and a value of m as approxi- 

 mately 1/1700 part of m' was thus obtained. 



Methods for determining the charge e were soon 

 devised. One of these, by Townsend, proved to be 

 basic to most of the subsequent determinations. In 

 Chapter XVIII we spoke as though all the molecules 

 of gas which rise from the electrodes of an electrolyte 

 were neutral. As a matter of fact, perhaps, one in a 

 million may carry a charge. The sign of the latter 

 depends upon the electrolyte from which the gas rises. 

 Now these charged molecules, when in air containing 

 water vapor, become nuclei of small drops, aggregating 

 to themselves water molecules and forming a visible 

 cloud. The natural assumption is that in such a 

 condensation the number of droplets is equal to the 

 number of unneutral gas molecules. The charge per 

 unit volume Townsend found by allowing the gas to 

 give up its charge to an electrometer (a calibrated 

 electroscope). The average radius of the droplets 

 he found by observing the time it took the cloud to 

 settle, under the action of gravity, through a known 

 distance and then applying Stokes's Law. The weight 



