cuthbert andrews cold-cathode ion x-ray tube, early 20th century.

In 1895, whilst studying the effects of passing an electrical current through gases at low pressure, German physicist Wilhelm Röntgen accidentally discovered x-rays. Within a year of his experiments the first radiology department was established at the Glasgow Royal Infirmary. The first x-ray image produced by Röntgen was of his wife’s hand.

Historically x-rays were discovered and experimented with using discharge tubes such as this one. Discharge tubes were invented by the British physicist William Crookes in the 1870s. The first models of x-ray tubes were cold cathode or Crookes tube which were popular until the 1920s.

This particular model was manufactured by radiographer Cuthbert Andrews (1882-1972) in the early 1900s. Andrews teamed up with C H F Müller - a German x-ray tube maker - to produce x-ray tubes from 1912. During the 1930's Andrews’ company became the sole British makers of x-ray tubes and once World War II broke out an alternative manufacturing site was established outside London.

X-rays are a type of electromagnetic radiation with the ability to penetrate partially opaque objects. Whilst they are similar to visible light, x-rays have a much higher energy which enables them to pass through and illuminate the inside of the human body. As the opaque bones are not penetrated by the x-rays, the skeleton is visible when the body is illuminated by the florescence produced in the tube. By placing a photographic plate the other side of the patient (an x-ray detector is used today), it was possible to view and capture images of the inside the body without surgically operating. This was a huge step forward for the field of medicine.

The tube works in roughly the following way:

  • Air is removed from the tube to reduce the pressure

  • High voltage is applied to the tube, the electric field accelerates the electrically charged ions and free electrons

  • Electrons collide with other gas molecules to create more positive ions – this chain reaction is called a Townsend discharge

  • The positive ions are attracted to the cathode, the electrons are attracted to the anode. These are the cathode rays.

  • As the air has been removed, the electrons travel the distance of the tube at high speed without obstruction. Due to their high speed they race past the anode and hit the glass. When this happens their energy level increases. Once the electrons return to their original energy level they emit light, this process is called fluorescence and causes the glass tube to glow.

This object is on display in the X-rays and Images display case at the George Marshall Medical Museum.