Atomicity and Quanta by James Jeans

By James Jeans

This can be the whole textual content of James Jeans's Rouse Ball Lecture given in 1925 at Cambridge college, and surveys the sphere of atomic and subatomic physics within the early days of quantum mechanics, with a short old point of view on size.

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Extra resources for Atomicity and Quanta

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Had disrupted the first atom it struck and yielded up its whole energy to this atom. If, however, radiation is to be compared to rifle-bullets, we know both the number and size of these bullets. We know, for instance, how much energy there is in a cubic centimetre of bright sunlight, and if this energy is the aggregate of energies of individual quanta, we know the energy of each quantum (since we know the frequency of the light) and so can calculate the number of quanta in the cubic centimetre.

A simple instance may bring the problem into focus. In 1901 Professor E. P. Adams attached a series of charged brass spheres to the circumference of a rotating wheel and found that these produced a magnetic field which alternated periodically as the spheres passed by a suspended magnetic needle. The electric field of course also varied periodically, and on calculating the Poynting Flux we can estimate the rate at which energy was radiated away from the rotating system. No physicist is likely to doubt that this radiation actually took place; in all essentials the method of its production was similar to that by which a field of radiation is generated in radiotelegraphy, and there can be no doubt as to the reality of the radiation here.

Bullets of radiation are supposed to hit electrons and to glance off at varying angles; part of their energy is imparted to electrons while the remainder passes on as a quantum of diminished energy, and therefore as a bullet of softer radiation. The supposition that the impact [ 57 ] between the bullets of radiation and the electrons conforms to the laws of conservation of energy and momentum leads to the formula SA. = —— (i — cos 0), for the increase in the wave-length A. of radiation which has been deflected through an angle 6.

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