All the technology needed to find the electron was available near the end of the 19th century.  The manufacturing of sealed glass instruments and tubes had reached a quality that could support low-pressure environments with the operation of a vacuum pump.  Around 1850 Heinrich Geissler invented a pump that could produce extremely low pressures.  He shaped glass into a tube and evacuated the air from within.  Then he filled the tube with a low-density, pure gas.  When a potential difference was applied to the electrodes at either end of the tube, a colourful electrical discharge was produced.  Further observations indicated that different types of gas emitted a unique and characteristic colour.  Geissler called these tubes gas discharge tubes.  The diagram below is an illustration of a simple gas discharge tube.

 

At this point, the electrons emitted in the tube (known as cathode rays ) became the subject of intense experimentation.

William Crookes proved that the emissions traveled from the cathode to the anode.  For this reason, gas discharge tubes are sometimes called Crookes tubes or, more commonly, cathode ray tubes (CRT).  Crookes placed a paddle wheel, which was free to rotate, in the path of the cathode rays.

 

When the rays hit the paddles, they caused the wheel to move along a track inside the tube.  This proved that cathode rays were composed of moving particles that had mass and momentum and were capable of doing work.





Cathode Ray: a free electron emitted by a negative electrode in a low-pressure environment

Crookes also demonstrated that cathode rays are deflected by a magnetic field.  Using the left-hand rule for moving electric charges in a magnetic field, he proved that the particles were negatively charged and were emitted by the cathode.



 

 

Another scientist, Arthur Schuster, used a set of external charged plates to demonstrate that the cathode rays were affected by electric fields.  The direction of deflection of the cathode rays further proved that they were, in fact, negative.

Based on these observations, cathode rays

  • travel from the negative electrode to the positive electrode 
  • travel in straight lines and cause shadows 
  • have momentum and energy to do work 
  • are deflected by magnetic and electric fields and have a negative charge


 


Read
Read "Cathode-ray Experiments" on pages 754 to 755 of the physics textbook. What was the evidence that the cathode rays were particles with charge and mass?