March 12, 1903] 



NA TURE 



45i 



where there is a pair of balanced forces, where a propelling 

 force acts all the time, just sufficient to overcome friction ; 

 e.g. a barge pulled by a horse, or a train drawn by a loco- 

 motive. When such a thing starts, the force is greater than 

 the resistance, and the speed accelerates; when it stops, the 

 resistance is greater than the propelling force; but when it 

 is going on at a steady speed, i.e. for the major part of its 

 journev, the force and the resistance precisely balance. The 

 resultant force acting upon it is nothing. It is obeying the 

 first law of motion. The barge moves, or the ship moves, 

 or the train moves, simply and solely because of its own 

 inertia. All the energy of an engine goes to generate heat 

 and to overcome resistance. There is no propulsion in that ; 

 when it is going at a steady pace the positive and negative 

 forces balance ; the body is subject to zero force and obeys 

 •the first law of motion. 



Now this property, a property analogous to inertia, 

 belongs also to electricity ; it was called self-induction, and 

 its laws have been made out for a long time, a law known as 

 Lenz's law, which says that any change in a current is 

 such as to oppose the motion. If you have a current of 

 certain strength any cause which increases that strength 

 calls out an antagonistic force. The force called out is 

 always antagonistic to any change in the current. When 

 a current is weakened, self-induction tends to make it persist 

 in retaining its old strength. It is a property precisely 

 analogous to inertia, and I now wish to suggest or maintain 

 that it is a property which actually is inertia. It depends 

 ■on a property which was first brought out mathematically 

 by considering the case of acceleration of a charged body. 



In a sphere charged with electricity, as long as it is at 

 rest, we have the phenomena of electrostatics ; directly it 

 is in motion we get the phenomena of current electricity. 

 A charged sphere in motion is a current, and we have to 

 realise that there is no other current but that ; a current is 

 surrounded by magnetic lines of force ; and when a sphere 

 or other body charged with electricity is put into movement, 

 a set of concentric circles of magnetic force surrounds its 

 path, giving rise to a magnetic field. That magnetic field 

 may seem extremely weak, but it is the measure of the 

 -current ; and whether weak or not, it is now believed to be 

 the only kind of magnetic field which exists. We are coming 

 to realise that there are three things — a charged body, a 

 charged body in motion, and a charged body in accelerated 

 motion ; the first gives us electrostatics, the second gives 

 us magnetism, and the third gives us two things, first the 

 evidence of inertia, and secondly radiation. Inertia and 

 radiation are not the same thing, but both are manifest 

 throughout the accelerated period. Inertia no doubt exists 

 all the time ; and instead of radiation 1 will use the more 

 general term of " light " — light being the best known form 

 of radiation. I will put inertia in a class by itself, because, 

 although it is only manifested when there is radiation, it 

 exists all the time. It does not depend on the speed, it is 

 constant, and may be taken to exist equally well when a 

 body is at rest. I want you to realise that just as there is 

 -no other electric field but that due to a charged body, so 

 there is no other current or magnetism except that due to a 

 charged body in motion, and there is no other radiation 

 except that due to an accelerated charge ; further, that one 

 kind of inertia is the inertia of the charge on a body, and 

 that probably, but not yet certainly, there is no other inertia 

 except electric inertia. 



With the time at our disposal it is impossible to give you 

 all the steps leading to this conclusion, I can only give you 

 a summary of the results. The idea of electric inertia as a 

 reality and as due to a moving charge took shape and form 

 in a magnificent paper by Prof. J. J. Thomson, of Cam- 

 bridge, which appeared in the Philosophical Magazine in 

 1881, one of the most striking productions in the recent 

 history of mathematical physics. It was a paper on the 

 properties of a moving charged sphere, and it showed that 

 a charged body possesses inertia because it is charged. 

 It is important to remember that a body when it possesses 

 a charge has, in addition to its ordinary mass, a sup- 

 plementary mass, as it were, proportionate to the square 

 of the charge, and inversely as the radius of the sphere 

 on which it exists; or, as we may also put it, it' is pro- 

 portional to the quantity and to the potential. No great 

 Importance was attached to the statement at the time be- 



cause of the difficulty of detecting any increase of inertia 

 due to the electric charge in the case of a sphere of appreci- 

 able size. The extra inertia would be excessively small and 

 impossible to detect if the sphere is of any perceptible size. 

 Even if the sphere is reduced in size until it is a mere atom, 

 and charged as highly as the atom can be charged, still the 

 inertia due to the charge would only be an insignificant 

 amount of the whole — not more than one hundred thousandth 

 part of the whole. That is to say, if you had one atom of 

 matter charged with the maximum quantity which it can 

 possess, and which you know in electrolysis or in chemistry, 

 and if the inertia of the atom itself was one hundred thousand 

 units, then when the charge was added it would be one 

 hundred thousand and one ; no important difference and not 

 experimentally to be detected. 



It depends, however, entirely how small the body is ; the 

 smaller the radius the bigger the inertia, due to the charge, 

 will be. For a long time nobody thought of anything 

 smaller than the atom, that was thought to be the limit, 

 hence electric inertia seemed to be no more than a matter of 

 mathematical curiosity. But about the year 1870 Sir 

 William Crookes called attention to the phenomena that 

 went on in vacuum tubes, and considered that the 

 kathode rays were matter in a " fourth state," neither 

 solid, liquid, nor gaseous. Sir William Crookes was not 

 believed, and was rather jeered at for speaking of matter 

 in a fourth state. However, the subject was investigated 

 by a great number of different people in this country and 

 in Germany ; and the result of these researches, in which 

 Prof. Schuster and many others, and notably Prof. J. J. 

 Thomson, engaged, has been to show that Sir William 

 Crookes was perfectly right ; that the matter in the vacuum 

 tube flying in these kathode rays is not solid, nor liquid, nor 

 gaseous, does not consist of atoms as had been thought pro- 

 pelled by the kathodes and flying through the tube and 

 causing phosphorescence where they strike, or X-rays, as 

 the case may be, but that they consist of something much 

 smaller than the atom, fragments of matter, ultra-atomic 

 corpuscles, minute things, very much smaller, very much 

 lighter than atoms — things which appear to be the founda- 

 tion stones of which atoms are composed. Thomson measured 

 the mass of these particles and found that they were of less 

 mass than the atom of hydrogen ; whereas the atom of 

 hydrogen had been the lightest body hitherto known. These 

 small corpuscles were about the one-thousandth of an atom 

 of hydrogen in mass, and he further made this important 

 observation, that whether hydrogen or oxygen or carbonic- 

 acid or any other gas was in the tube, the particles into 

 which these substances seemed to be broken up by electric- 

 action were identical and independent of the nature of the 

 gas in the tube. That is to say, the things shot out by the 

 kathode did not depend upon what gas was in the tube ; 

 they seemed to be fragments of the atoms of the gas, but 

 they were the same fragments in each case. This at once 

 suggested the hypothesis, not yet by any means completelv 

 verified, that all atoms of matter may be composed of these 

 same corpuscles, or electrons as Dr. Johnstone Stoney had 

 called them. Dr. Stoney had a habit of being in the van 

 and of naming things before they had been discovered; 

 thus they were called electrons long before they were known 

 to exist separately — only the name belonged to the charge 

 of an ion in electrolysis — a charge associated with matter ; 

 but in a vacuum tube these same charges are detached 

 from the atom and fly free, a thing previously unheard 

 of. In liquid conduction the charge and the atom 

 travel together, they are inseparably associated ; at an 

 electrode or solid conductor the electron or charge is handed 

 on to the metal and goes along the wires by some other 

 means, but while they are travelling they are definitely 

 united or attached to atoms all the time, although passed 

 from hand to hand ; in a gas it is not so, for it is just as if 

 charges had been knocked off, charges of electricity dis- 

 sociated from the matter, disembodied charges or electric 

 ghosts flying through the tube at a tremendous speed. It 

 was not only possible to measure the mass of the particles, 

 it was also possible to measure their speed, and their speed 

 was found to be something comparable to that of light, about 

 one-thirtieth or sometimes even one-tenth of the velocity of 

 light. Anything moving with that prodigious speed of 

 several thousand miles per second must have a great amount 



NO. 1 74 1, vol. 67] 



