Lesson 42 - A Short Review

Thomas Edison, famous (at least in America), for inventing the light bulb, _{See Note 1}made many discoveries before he completed his task of lighting the path of the world. Along the way, he incidentally noted that if a filament were energized within a vacuum, that after time, a "shadow" would be left on the inside of the glass, which resembled the shape of the filament. He surmised from this, that within a vacuum, particles (we now call them electrons) were emitted around the wire, forming a cloud, or SPACE CHARGE. This effect became known as the EDISON EFFECT, which is the basic operating theory behind all vacuum tubes.

     Later, J. Ambrose Fleming invented the FLEMING VALVE, when he noticed that a second ELEMENT, or ELECTRODE within the vacuum along with the filament, but not touching it, electricity would flow through the vacuum and be collected on the second element. The second element was called a PLATE. He further noted that electricity would flow from the filament to the plate, but not in the opposite direction. The Fleming valve was later dubbed the DIODE, because it has 2 elements inside the vacuum - the filament and the plate.

The AUDION came about when Lee DeForest, In 1906, added a 3rd element between the two. This third element, a control grid, allowed one to electronically control the output of the tube based directly upon the input. This was the birth of Amplification. The term AUDION was later replaced by the term TRIODE, as the tube has 3 elements within the vacuum.

The reason why a tube works is because the CATHODE is heated to a point of THERMONIC EMISSION, forming a SPACE CHARGE or cloud of electrons, which is attracted to the positive charge on the PLATE or ANODE. The Cathode ( K ) of a tube can be either directly or indirectly heated.


Diode tubes only allow electrical current to flow in one direction. By using a diode tube we can change, or RECTIFY, Alternating Current into Direct Current.

An X / Y plot of Voltage vs. Current produces the CHARACTERISTIC CURVE of a device. We looked briefly at the characteristic curve of a DIODE tube. Now let's look, for a moment, at the curve for a resistor.

Remember on page 8, we discussed Ohm's law. We used a 10 Ω resistor for an example. When we applied a 100 volt source to the resistor, 10 Amps would flow through it. What happens, though, if we reduce the voltage to 10 volts?



10 Volts / 10 Ω = Click for Answer



Now let's try another voltage value.

50 Volts / 10 Ω = Click for Answer



Finally, what happens if we apply a Zero volt source to the resistor?

0 Volts / 10 Ω = Click for Answer
If we plot this change on a chart, we will find that a resistor has a characteristic curve which equals a straight line. Not all resistors will be an exact 45° angle. The angle of the line will depend on the value of the resistor. However the characteristic curve of all resistors will be a straight line if plotted voltage vs. current.





Note 1:

Not to incur the wrath of Edison haters or Tesla enthusiests - Tesla is indeed the inventer of many things - including the AC power that the Edison Light bulb presently runs on. However, Tesla did not invent everything, and absolutely did not invent the vacuum tube or light bulb. Of noteworthy mention is the work of Swan in England, upon which many of Edison's experiments were built. In the end - we all build on the work of others, and stand on the shoulders of giants. Tesla was a giant, as was Swan, Edison, Fleming, Deforest, Ohm, and many others, and all built on knowledge obtained through the works of others. Dare I say Newton?