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Cathode Ray Oscilloscope

An oscilloscope is a device to plot a graph of an electrical voltage.  This description does not tell us how useful this instrument really is.  The reason for its usefulness is that many signals can be converted into electrical voltages: sound using microphone, light using photodiodes, temperature using thermistors, etc.  And it is graphs of sound waves, light intensity, temperature variations, etc. that we really want.

A  cathode ray oscilloscope (CRO) is an oscilloscope that works by using an electron beam.  (Cathode ray is a historical name for electron beam.)  Lets look at how an electron beam can plot a graph.

An electron beam can be created by emitting electrons from a hot filament, then accelerating and focussing these electrons.  This setup is called an electron gun - literally a gun that shoots electrons.  This electron gun is housed at one end of a long glass tube called a cathode ray tube (CRT).  The other end of the tube is a flat area covered on the inner wall with a layer of fluorescent material.  If an electron beam hits this material, it glows.  This wall is called the fluorescent screen.  Because the electron gun can focus a beam of electron into a very small area, we see a small bright spot where electrons hit on the screen.

So the electron beam has drawn a dot on the screen.  If we can somehow move this dot around, we can even make the beam draw pictures on the screen.  This is actually how the television works, though such televisions are increasingly replaced by LCD.

We shall first draw a line with the electron beam.   One way is to make the electron beam move up and down very quickly.  Then the dot moves up and down so fast that our eyes would see a line.  A simple way to make the beam move is to bring an electrical charge near the beam.  In this picture, there is one metal plate above the beam and one below.   

These two plates are connected to an alternating voltage (e.g. a.c. generator, but usually some electronic device).  The plates become charged alternately with positive and negative charges.  When the top plate is positive (and the bottom plate is negative), the beam is attracted up.  When the top plate is negative (and the bottom plate is positive), the beam is repelled down.  This keeps repeating, so the spot on the screen moves up and down rapidly and shows a vertical line.    

Now that we know how to draw a vertical line, we can also draw a horizontal line by putting plates to the left and right of the beam, as shown by the x plates in this picture.

Voltages on the y plates move the beam up and down.  Voltages on the x plates move the beam left and right.  If we can create the correct voltages, we can draw anything we like on the screen.  

To draw a graph of a sound wave, the y plates should be connected to a microphone.  If you sing into the microphone, the microphone converts your sound wave into voltages that move the dot up and down.  The oscilloscope would come with electronic circuits that can produce a voltage on the x plate to sweep the dot from left to right at constant speed.  (This voltage is called the time base.)  The combined effect of these two voltages on the x and y plates is to produce a graph of the sound wave on the screen.