4 THE QUANTUM [1.2
I S
the inea.surern.ents of Kurlbaum and of Lummer and Pringsheim
was
h — 6 55 x 10- 27 erg sec.,
a value which is in remarkably good agreement with later and
more accurate determinations by various methods. The value
adopted in this book is
h = 6-558 x 10- 87 erg sec.
It will be noticed that h is a quantity of the dimensions of
energy multiplied by time, i.e. of action, as the term is used in
applied mathematics in connection with the Principle of Least
Action.
The fundamental relation of Planck’s theory may obviously
be written in the form
— — nh 1:2
v
v being the frequency. The equation may also be written
Et = nh 1:3
where r is the period of the vibration. This may be interpreted
by regarding E/V or Er as the action, and h as an element of
action. From this point of view, which agrees closely with
that adopted in the later developments of the subject, the
quantum of action, h, is a universal constant, a true atom of action.
We may gain some modicum of satisfaction from the idea that
action is atomic, though as Jeans has remarked “ an attempt to
imagine a universe in which action is atomic leads the mind
into a state of hopeless confusion ! ”
An alternative mode of regarding the quantum constant may
be mentioned here. J. W. Nicholson,* in an investigation of
the nebular and coronal spectrum, suggested that the angular
momentum of an atomic system might be equated to nh/2n.
Here h/271 appears as a natural unit of angular momentum, the
physical “ dimensions ” of this quantity being the same as those
of action.
2. Photo-electricity
In 1905 EinstemJ made a notable suggestion which has
proved fruitful in predicting experimental results although it
raises many grave difficulties. This was the hypothesis of the
existence of “ light quanta,” an idea which has also been developed
by J- J* Thomson in his unitary theory of light. According to
* J. W. Nicholson, Monthly Notices, R.A.S., vol. 72, p. 677, 1912.
t A. Einstein, Ann. d. Physik, vol. 17, p. 132, 190 е ,: vol. 20, p. 199,
1906.
