ical Laboratory, to
test such instruments and report on their accuracy. International
conferences have been held for the purpose of reducing these units to as
small a number as possible so that people may be able to trade less
wastefully and more conveniently, so that also the barriers between
peoples may be broken down and the interchange of ideas as well as of
materials may be made more easily. Without an arrangement of this kind
it would be impossible to carry on industrial life in which use is made
of electricity. It would be as difficult as to hold a market without the
use of weights and scales, more difficult, in fact, since anyone can
estimate the size of a piece of cloth or the amount of corn in a sack,
but no one has a natural sense by which he can estimate an amount of
electricity.

In just the same way energy can be measured as a quantity in terms of a
fundamental unit. The discovery that this was so was made by Joule and
others towards the middle of the nineteenth century, and lit the road
for further advance as a dark street is lit by the sudden turning-up of
the lamps. All modern industry rests on this principle. We are now so
accustomed to the idea that energy is a quantity that we can hardly
realize a time when it was merely a vague term. If we want an
illustration of how thoroughly we have grasped this idea let us remember
that when we pay our electric-light bill we pay so much money for so
many units of energy supplied; for so much energy, let us note, not for
so much electricity, since we take into account not only the actual
amount of electricity driven through our house wires, but also the
magnitude of the force which is there to drive it. Energy exists in many
forms: energy of motion, heat, gravitational energy, chemical energy,
radiation, and so on. In the transformations of energy which are
continually occurring in all natural processes, there is never any
change in the total amount of energy. This is the famous principle of
the Conservation of Energy. Sometimes it is stated in the form
'Perpetual motion is impossible'.

One of the most important forms of energy is radiation. The constant
outpouring by the sun of energy in this form is vital to us. The fact
was obvious long ago and that is one of the reasons why light and heat
have interested students of science in all ages.

There exist then three main subjects of study--matter, electricity, and
energy. These themselves and their mutual relat