Cathode rays are produced in discharge tubes at very low pressures

The discharge tube

• A discharge tubeA sealed glass tube containing gas at low pressure, with a cathode and anode at each end. When a high voltage is applied, the gas conducts and glows. is a sealed glass tube containing gas at low pressure with a metal cathode at one end and a metal anode at the other.
• A high voltage (several kV) is applied between the cathode and anode using an induction coilA device that produces a very high alternating voltage from a low-voltage DC supply, used to create the large p.d. needed across discharge tubes..
• At atmospheric pressure, air is a good insulator and the tube does not conduct.

What happens as pressure drops

• As the gas pressure is reduced using a vacuum pump, the behaviour of the tube changes in a characteristic sequence:
• At moderate vacuum, the gas begins to conduct. The applied voltage ionises gas molecules, knocking electrons free. These electrons collide with other gas molecules, releasing more electrons in a chain reaction.
• The positive ions left behind are attracted to the cathode, and free electrons are attracted to the anode. This movement of charge is the current through the tube.
• The gas glows because excited atoms emit photons as their electrons return to lower energy levels. Different gases produce different colours.
• At very low pressures (below about 0.01 mmHg), there are too few gas molecules to produce a visible glow. Instead, a greenish fluorescence appears on the glass wall opposite the cathode.
• This fluorescence is caused by cathode raysStreams of electrons emitted from the cathode of a discharge tube at very low pressures. Originally observed as fluorescence on the glass walls. striking the glass. The rays travel in straight lines from the cathode.

Properties of cathode rays

• Cathode rays travel in straight lines (shown by the sharp shadows cast by obstacles placed in their path).
• They are deflected by electric and magnetic fields, which confirmed they carry negative charge.
• They cause fluorescence when they hit certain materials, such as glass or zinc sulphide.
• The key part is that these observations eventually led to the identification of the electron as a fundamental particle.