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Electron Beam

When you connect a battery to a light bulb, the bulb lights up.  Electrons flow through the wires and carry electrical energy to the light bulb.  What if we can somehow pluck these electrons out of the wires and throw them into empty space?  That may sound strange, but electron beams travelling through a vacuum is actually a very useful thing.  It has given us X ray, diodes, transistors, amplifiers, oscilloscope and television.  Some of these are replaced by semiconductors today, but electron beams are still widely used.

The simplest way to remove electrons from the wire is to use the filament in the light bulb.  When the filament lights up, it also gets very hot.  Then electrons will just jump out of the filament into the space around it.  This process is called thermionic emission.

 
To make the electrons really useful, we must turn it into a beam travelling through a vacuum.  A simple way is to just use a huge light bulb with the filament on one side, and a metal plate on the other side.  A battery is connected to the filament to heat it up and emit electrons.  A second battery is connected between the filament and the metal plate, with the plate as positive pole.  The plate attracts the negative electrons, which then travel across the space in between as a beam.

However, this is not a very nice beam.  It is broad and random.  For it to be useful, we really want it to be a sharp, narrow beam.  To do this, we need to accelerate and focus the electrons.  One way to do this is to replace the metal plate with a metal ring, and the second battery by a high voltage supply.  This ring will attract the electrons strongly.  When the electrons zoom through the ring, they get focussed into a narrow beam.

The obvious question then is: what do we want to do with this beam?  There are many uses, as mentioned above.  We shall look at how it is used in the oscilloscope.