4.1 Introduction

The early X-ray tubes contained an amount of residual gases and consequently
were known as "gas" tubes. They were inefficient and unreliable and
susceptible even to small changes in the gas temperature or pressure. They
were only capable of producing X-rays up to energies of about 130 kV.

In 1913, William David Coolidge developed a new X-ray tube. He found that
tungsten had certain characteristics that made it a better target material than
platinum. He first produced a ductile tungsten and then succeeded in vacuum
casting copper around a tungsten disc. This greatly increased the heat
conductivity of the anode which allowed for increased energy and a greater
quantity of penetrating X-rays. Tungsten is now widely used for targets except
in tubes built for special applications.

Dr. Coolidge's greatest contribution was discovering that an X-ray tube could
be made to operate with consistency when a spiral filament of tungsten wire
heated to incandescence by means of electric current was used in a vacuum tube.
His first models operated at voltages of 140 kV-200 kV. These were the
forerunners of the modern X-ray tube.

In order to produce even higher voltages, Dr. Coolidge developed in 1922, a
sectional type of X-ray tube using what he called the Cascade principle.
Voltages were applied to each section of the tube accelerating the electrons in
steps. Using this principle he was able to produce a million volt X-ray tube.

Development of other necessary components of an X-ray apparatus was also
continuing. Earlier shock-proof equipment had used oil in the casing as an
insulator, however, it was found that Freon gas and later sulphur hexafluoride
(SF6) pumped in under pressure was more efficient.

Concurrently, new types of iron core transformers were developed as well as
those which eliminated the iron core used in the centre of the copper wire
coils. New forms of rectifiers allowed for a greater X-ray output using
alternating current. More efficient electrical circuits were developed and
scientists, who were more knowledgeable of the structure of the atom and of the
various particles of which it was formed, were completely involved in the
production of modern X-ray apparatus.