EXPLORING THE ATOM 



The apparatus through which this seemingly 

 miraculous feat has been accomplished, is known as 

 the electroscope. It is an instrument which constant- 

 ly serves the student of radioactivity. Even as com- 

 pared with the spectroscope and bolometer and 

 radiometer, it is an apparatus of extraordinary deli- 

 cacy. 



The spectroscope, as we have seen, reveals infini- 

 tesimal traces of a substance. It can show the pres- 

 ence of the minutest quantity of a gas in a tube that 

 in ordinary parlance would be said to be absolutely 

 empty. But even the best vacuum that the physicist 

 is able to produce contains many millions of gaseous 

 molecules to the cubic inch. So after all the spectro- 

 scope is dealing with a vast swarm of molecules when 

 it performs its most delicate feats. But the electro- 

 scope, as just intimated, is capable, under certain 

 circumstances, of detecting the presence of a single 

 atom. The tests that it can apply are estimated to be 

 500,000 times more delicate than the finest tests of 

 the spectroscope. 



In principle the electroscope is simplicity itself. 

 It consists essentially of two bits of gold leaf sus- 

 pended loosely together. If these gold leaves are 

 electrified their mutual repulsion holds them apart. 

 But if the electricity is discharged they fall together. 

 Under ordinary conditions perfectly dry air is a non- 

 conductor of electricity; therefore a charged electro- 

 scope shows its leaves spread apart. If the atmos- 

 phere is electrified, or as the technical phrase is, 

 ionized, it becomes a conductor and permits the elec- 

 tricity to escape. 



