POPULAR SCIENCE 625 



Recent investigation has shown, however, that we cannot add 

 or subtract any amount of electricity we please. There is a 

 certain limiting amount below which we cannot go. This 

 amount is, in fact, identical with that carried by a single elec- 

 tron. Electricity, or, more properly speaking, electrical 

 charge, is built up of certain natural units, each unit being 

 identical in magnitude with the charge on an electron. 

 That is, electricity has a discrete structure, just as matter has. 



The discovery of electrons and their r61e in electrical 

 phenomena was made by Sir J. J. Thomson. It was shown 

 that electrons constitute the cathode rays, a stream of very 

 fine particles which is produced when an electric discharge is 

 passed through a gas at low pressure. The stream of electrons 

 is made manifest by a glow in the tube, the glow itself being 

 due to the collisions of electrons with gas molecules, the latter 

 suffering ionisation. The stream could be deflected by a 

 magnetic field, showing that the particles composing it were 

 electrically charged, and the direction of the deviation indi- 

 cated that, on the usual convention of positive and negative 

 electricity, the electrons were negatively charged. In addition 

 to the cathode rays, in which the electrons are produced from 

 the matter composing the electrodes, we likewise meet with 

 their production from radio-active elements, from the alkali 

 and alkaline earth metals, and in general from any material 

 which is " excited " in a suitable manner, e.g. by exposure 

 of the material to short wave or ultra-violet light. It seems 

 natural, therefore, to regard the electron with Sir J. J. Thom- 

 son as " one of the bricks of which atoms are built up." 



The existence of this natural unit of negative electricity 

 having been demonstrated beyond doubt, the question natur- 

 ally arises — is there a similar unit of positive electricity ? So 

 far, the answer is in the negative. We have not yet encoun- 

 tered a natural unit of positive electricity which is indivisible 

 and capable of independent existence as in the case of the 

 electron. At the present time positive electricity really denotes 

 absence of a certain amount of negative electricity, i.e. absence of 

 a certain number of electrons. Thus, it is possible to remove 

 an electron from an atom, say an atom of sodium, leaving us 

 with a positively charged atom-residue or ion. The quantity 

 of positive electricity on the ion is necessarily equal in amount, 

 though opposite in sign, to that of the electron itself. A very 



