Man's physical universe

xanabras

UNIT VII

SECTION 9

ELECTRICAL CONDUCTION IN GASES IS OF TREMEN-

DOUS THEORETICAL AND PRACTICAL IMPORTANCE

There is probably no subject in all science which better illustrates the

importance to the entire world of research in pure science, than does X rays.

Within three months after Roentgen's fortuitous discovery, X rays were being

put to material use in a hospital in Vienna in connection with surgical operations.

Had Roentgen deliberately set about to discover some means of assisting

surgeons in reducing fractures, it is almost certain that he would never

have been working with evacuated tubes, induction coils, and the like, which

led to his famous discovery. — F. K. Richtmyer.

Introduction.

The discovery that electricity could be conducted by highly rarefied

gases led to the development of Crookes tubes. The study of the

phenomena of Crookes tubes led in turn to X-ray tubes. These were

very important discoveries, for they paved the way for the great

advances of the past twenty-five years which have revolutionized

physical science. In this Section these discoveries will be described,

leaving only their tremendous significance to be studied in the next

Unit.

Air Must Be Ionized to Become an Electrical Conductor.

Under ordinary conditions air is a poor conductor of an electric

current, but under certain conditions, as we have seen, it will act as

a conductor; thus a current passes between the carbons of an arc

light or between the terminals of an induction coil. When a spark

passes between two electrodes separated by an air gap, it is because

the air is ionized. The charge acquired by molecules is produced in a

number of ways, but in every case electrons are driven out of atoms,

thus leaving them positively charged, while the electrons themselves

may act as negative ions or may become attached to neutral molecules

or dust particles, thus giving them a charge.

The electroscope is an excellent instrument for determining the

ionizing power of different agencies, because the charged leaves of the

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