8o 



NATURE 



[March 17, 192 1 



to be found everywhere and in every body. Since 

 then the measurements of these quantities have 

 been repeated many times with increasing- skill and 

 understanding-. They have reached their present 

 high-water mark perhaps in the experiment of 

 Millikan at Chicago, who gives as the value of the 

 charge in electromagnetic units 6 = 1-591x10-20^ 

 the mass being 0-900x10-27 gram, or 1/1850 of 

 the mass of the hydrogen atom. 



So we arrive finally at an accurate comparison 

 of these unique and fundamental units of Nature 

 with the units which we ourselves have chosen for 

 our convenience, and without, of course, any con- 

 sideration of the former. We infer from experi- 

 ments such as those of Kaufmann and of Bucherer 

 that the energy of the moving electron may be 

 considered to exist wholly in the form of electro- 

 magnetic energy, such as is necessarily present 

 when an electrical charge is in motion ; and that 

 its mass is in this way perfectly accounted for. 

 But this conclusion sets a limit to the size of the 

 electron, and we must assume that its radius, if 

 its form is spherical, is very small compared with 

 the rafdius of any atom. Also, as the velocity of 

 the electron approaches that of light, its mass in- 

 creases ; imperceptibly at first, but in the end very 

 rapidly. 



Why, we may well ask, have these measure- 

 ments of charge and mass never been made 

 before? The electron is everywhere : the transfer 

 of electricity from place to place consists always 

 in the transfer of electrons. The electric current 

 is a hurrying stream of electrons : all our elec- 

 trical machinery concerns itself with setting them 

 in motion, with giving them energy and again 

 withdrawing it. In the processes of electrolysis 

 the electrons are handed to and fro. Everywhere 

 they fill the stage; why have we not hitherto 

 noticed their qualities, which so far can be ex- 

 pressed so simply? 



The answer is that we have never, until recently, 

 been able to make them move fast enough in 

 spaces sufficiently empty of air or other gases. 

 It is only when an electron has a sufficient speed 

 that it can escape absorption in the atoms which 

 it must be continually meeting. Unless an elec- 

 tron has a speed exceeding about one three- 

 hundredth of the velocity of light — that is to say, 

 such a speed as it acquires in falling through a 

 potential of a few volts — it sticks to the next atom 

 it runs up against : even with ten times that speed 

 it can move only a fraction of a millimetre through 

 air at ordinary pressure before it loses its velocity, 

 and, therefore, its power of going through the 

 atoms. When Crookes first saw the cathode-ray 

 stream in full course, it was because he had re- 

 duced the number of gas molecules in his bulb 

 to such an extent that an electron could fly in a 

 straight line from end to end of the bulb w'ithout 

 going through more than a hundred atoms or so, 

 and the induction coil had given it quite enough 

 speed to do that without turning out of its course, 

 no matter what sort of atoms they were. Inci- 

 dentally, since atoms can be traversed in this way, 



NO. 2681, VOL. 107] 



we naturally think of an atom as a very empty 

 affair. 



Electrons flying still faster than in the discharge 

 tube are found to constitute a part of the radia- 

 tion from radioactive substances. Some of the 

 /8-rays have velocities nearly equal to that of light 

 and can pass through millions of atoms before 

 their energy is spent. In open air a )8-ray may 

 have a course of metres in length, though it is 

 generally broken by encounters with traversed 

 atoms into a path full of corners and irregulari- 

 ties. 



It is speed which gives separate existence to 

 the moving electron : and speed which also betrays 

 its presence to us. For, on its way, the electron 

 here and there chips away another electron from 

 an atom which it is crossing and leaves behind it 

 a separation of electricities which may after- 

 wards influence chemical action, as in the case of 

 the phosphorescent screen or photographic plate, 

 or provide a current for the ionisation chamber. 

 We do not know exactly how this removal of elec- 

 trons is effected, nor why some atoms part with 

 electrons more easily than others, so that the flying 

 electron loses less energy as it goes through : 

 there is much that is obscure in the whole process. 

 But it gives us a ready means of observation, 

 without which, indeed, our knowledge of the elec- , 

 tron would be far less than it is. 



These electrons which are so made manifest by 

 speed form but a minute fraction of the whole 

 number existing. They are to be found in every 

 body, and in every atom of every body. They 

 form one of the elements of construction of the 

 atom, and it is one of the most immediate aims 

 of present research to find in what way they are 

 built into atomic structure. In every atom there 

 are certain electrons of which one can be removed 

 at the cost of an amount of energy of the order 

 of lO"^^ cg^s. The potential through which an 

 electron must fall so that it acquires this energy 

 is of the order of a few volts. There are other 

 electrons within the atom which are intrinsically 

 far more difficult to remove. On the other hand, 

 some atoms — for example, those of a metal in the 

 solid or liquid condition^ — have each one or more 

 electrons which are little more than hangers-on, 

 and are, indeed, removed with very little trouble. A 

 block of pure metal is full of such loosely bound 

 electrons, so that if an electric potential differ- 

 ence is maintained across the block an electron 

 flow or electric current is produced. The metal 

 "conducts." 



At sufficiently high temperatures all bodies 

 become conductors ; we must imagine that the 

 violent thermal agitation shakes electrons free 

 from their ties to the atoms even when at low 

 temperature the bonds ordinarily remain unbroken. 

 At a high temperature, too, the electrons acquire 

 high velocities as they move to and fro with their 

 proper share of heat energy. At the surface of 

 the hot body the electrons may break away ; and 

 hence the "thermionic emission" investigated by 

 O. \\'. Richardson. So copious is this supply of 



