8 THE PRESIDENTIAL ADDRESS 



scores or hundreds or thousands of vohs. That is done by trans- 

 ferring some of the crinohne electrons from one to the other plate : it 

 involves only a minute supplementary separation, which disappears 

 when the condenser is discharged. In every atom we have a per- 

 manent separation of electricities ; the protons and electrons look at 

 one another, so to speak, across an immensely greater dielectric gulf 

 which no laboratory operation ever causes them to bridge. That 

 is why every atom is a magazine of energy, the quantity of which 

 (mc^) is proportional to the atom's mass. 



Any of the usual operations of the electrical engineer, such 

 as charging and discharging a condenser or a storage battery, 

 or driving a dynamo and conducting electricity from it to a 

 distant station where it can actuate a motor or heat the filaments 

 of lamps to incandescence, may be described as the setting up and 

 the breaking down of a comparatively small extra difference of 

 potential between the opposed electricities in some of the atoms of 

 the engineering plant. In every process of industrial electricity, 

 on whatever scale, what happens is a temporary enlargement of the 

 potential difference which always exists between electrons and 

 protons, and then a return to what may be called nature's status 

 quo. But those supplementary differences of potential which the 

 engineer first superimposes and then allows to disappear are ex- 

 ceedingly small, even at their greatest, in comparison with the 

 gigantic difference which the normal condition of the atom itself 

 involves. 



A notable event of the year is the strong evidence which 

 Dr. Chadwick of the Cavendish Laboratory has found for the 

 existence of what is called the neutron — a type of particle in which 

 an electron and a proton are associated in particularly close 

 juxtaposition. There is a like close association between electrons 

 and protons in the nucleus of any heavy element, but it had not 

 previously been discovered in a single isolated pair. Twelve years 

 ago Lord Rutherford conjectured the existence of such a particle 

 and described the properties it should possess. Its excessive small- 

 ness and density, together with its lack of an external electric field, 

 give it a unique power of penetrating matter. It is too slim to be 

 confined under pressure in any vessel : it will simply slip through 

 the walls. The normal hydrogen atom has the same two constituents, 

 one proton and one electron, but in nothing like the same intimacy 

 of association, for the hydrogen atom wears its electron as a bulky 

 crinoline which confers on it an immensely greater volume. The 

 neutron, on the other hand, may be said to have taken the crinoline 

 off, folded it up and put it in its pocket. Not to be too fanciful, we 

 may at least describe the partners as clasping one another so tightly 

 that the electron has ceased to be a fender ; none the less as a unit 



