CONTEMPORARY ADVANCES IN PHYSICS 321 



spinning atom did not make an earlier entry upon the scientific stage. 

 Perhaps some of you will remember hearing the words "vortex atom" 

 and "vortex theory" which used to be so prominent in physics, and 

 will take the spinning atom of today for a lineal descendant of those 

 vortices of old. If this were correct, we could trace back the ancestry 

 of the spinning atom for about three hundred years; but I think that 

 it is not correct. The vortices of which Descartes and Malebranche 

 were dreaming three centuries ago were more like whirlpools of stream- 

 ing particles, and the vortices which were imagined by Helmholtz 

 and Lord Kelvin some fifty years ago were also whirlpools, but they 

 were whirls in an idealized continuous frictionless fluid. Let us pause 

 for a moment to notice how the attitude of physicists has altered in 

 these fifty years! Kelvin and Helmholtz began with the idea of an 

 aethereal fluid pervading the whole of space, and valiantly tried to 

 represent the atoms as whirlpools in that fluid ; but we have long since 

 discarded that aether, and our spinning atoms and other elementary 

 particles are small delimited rotating bodies voyaging in a void. 



It is nor therefore the vortex which I will introduce to you as the 

 ancestor of the spinning atom, but rather the "Amperian whirl" as it 

 still is sometimes called. You remember, of course, how Ampere in 

 1820 made a very great achievement which for the purposes of this 

 talk I will divide into three. First, he discovered the fact that an 

 electric current flowing in a circuit is equivalent to a magnet. Next, 

 he worked out the mathematical laws whereby, given a current and 

 the circuit in which that current is flowing, we may calculate the 

 strength or the moment of the equivalent magnet. I will write down 

 the formula for the case of a current i, flowing in a plane loop of area A : 

 the magnetic moment of the equivalent magnet, /x, is given thus: 



fj. = iAjc. 



Here c is a factor of transformation which we are now obliged to 

 employ because we habitually use, in atomic physics especially, a 

 unit-system different from Ampere's. The third part of the great 

 achievement was this: Ampere founded what remains to this day the 

 theory of magnetism, by presuming that the individual atoms of any 

 magnetizable substance are themselves little magnets, and that the atoms 

 are magnets because they have little whirls of current in them. 



This notion — the notion that atoms are magnets, and that they are 

 magnets because they have internal circulating currents — is the true 

 forerunner of our present conception of the spinning atom. It is, 

 however, only a very primitive form of the modern conception, and 

 there is much to be added to it. First of all and above all, there is 



