Raoioactiviiy

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Radioactivity

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charcri.'? nl any infractions of the rules which
it promu^iUej. It ksucs broadcasting li-
censes and determines the power to be used
by each broadcasting station. There are a
number of powerful stations in the United
States each operating with 50,000 watt
power. A North American Regional Broad-
casting Agreement was entered into in 1937,
by the United States, Canada, Mexico, Cuba,
Santo Domingo and Haiti, and was ratified
by the U. S. Senate in 1938. It provides
the instrument whereby service may be af-
forded by stations in the various countries
with the minimum of interference from other
stations. Without such co-ordination the
increasing number of stations was progress-
ing toward a chaotic condition in Ameri-
can broadcasting. In Europe, broadcasting
difficulties were to a considerable extent
regulated by the Pact of Lucerne, 1933,
under which 28 countries of Europe, Africa
and Asia Minor signed rules concerning
time allotments, frequency and power; but
these regulations and subsequent improve-
ments became badly disrupted when war
occurred in Europe in 1939, and naturally
the war of propaganda on the air inten-
sified, as did efforts by each nation to over-
power and produce interference with its
enemy's radio. The voices of leading states-
men, correspondents, and commentators of
Europe are frequently heard in millions of
American' homes.

Radioactivity, the property, possessed by
certain bodies of emitting, spontaneously,
characteristic rays, invisible to the eye and
capable of penetrating substances impene-
trable by ordinary light. Becquerel, in 1896,
while investigating various properties of
phosphorescent bodies, discovered that com-
pounds of uranium, w,hen left in the neigh-
borhood of a pKotographic plate in a per-
fectly dark room, affected the plate, even
though it were wrapped in black paper. In
addition to this photographic action through
a covering opaque to ordinary light, it was
found that uranium compounds caused the
air in their vicinity to become 'a conductor
of electricity; so that, for example, a charged
gold-leaf electroscope placed near a small
quantity of uranium rapidly lost its charge,
exactly as if the air had been made a con-
ductor by the passage of Rontgen rays. Pro-

. It ai&o cceiofscs j fessor and Madame Curie proved, in 1898,
phon^ and telo-1 that there are only two ordinary, well known
elements which possess in appreciable degree
this property of radioactivity, and these two
are those which have the heaviest atoms,

namclj uranium and radium. The radiations
from radioactive substances have been called
Becquerel rays, after their discoverer. They
consist of three types, which have been
named the a (alpha), ft (beta), and 7 (gam-
ma) rays respectively.

The /? rays, which are in many respects the
simplest, are rays similar to cathode rays of
high velocity (see VACUUM TUBES), and con-
sist of negatively charged particles (negative
'electrons') whose mass is i/iS4$th of that
of an atom of hydrogen, travelling with stu-
pendous velocities which range from i/ioth
the velocity of light up to very close to that
velocity, namely 186,000 m. per second. They
can be deflected by a magnetic or an electro-
static field in a similar manner to the cathode
rays. The1 7 rays are about a hundred times
more penetrating than are the j3 rays and are
not deflected by a magnetic field, however
strong. They are entirely distinct from the
(3 rays, since, instead of being projected
corpuscles, they are, like Rontgen rays, ether
radiations of very high-frequency, i.e., of very
short wave length. These ether radiations ac-
company the production of the ft rays in
much the same way that the Rontgen rays
are formed in a vacuum tube by the sudden
stoppage of cathode rays by a suitable object.

The a rays are, however, the most import-
ant, representing, in general, as much as 99
per cent, of the total energy radiated. They
consist of projected particles, positively
charged, of a mass the same as that of the
helium atom. Indeed, they are the same as
the nucleus of the helium atom, and become
neutral helium atoms as soon as their two
free positive charges (positive electrons)
have been neutralized by the attachment of
two negative electrons or ft particles. The
most remarkable feature of radium is the way
in which it emits energy. Its 7 radiations
are exceedingly penetrating, one or two milli-
grams of fairly pure radium enclosed in a
leaden tube with walls J4 inch in thickness
discharging quite rapidly a gold-leaf electro-
scope held anywhere near it. Radium prep-
arations are also self-luminous, and possess
extraordinary power of bringing about
chemical action: thus, when dissolved or
suspended in water, they set free oxygen and
hydrogen; they cause elements to change
into their allotropic forms — e.g., ordinary to