Let me present you some experiments with transistors.
Please look at these two circuits:
The upper circuit has two NPN transistors (for NPN I have used the model BC547), while the lower circuit has two PNP transistors (for PNP the model BC557). They form the so-called Darlington pair.
The experiment can be also carried out with only one transistor per circuit, but with two transistors it is more effective. The number of transistors doesn’t change anything principally.
We now take a vinyl gramophone plate, a thin-walled glass, and a piece of woolen and silk fabric. We rub the vinyl plate with the woolen cloth and bring it close to the loose end of the wire of the lower circuit ( we will call it Minus-circuit). We will see that the LED will light up for a moment. It will also light up if we bring it close to the wire’s loose end of the upper circuit ( We will call it Plus-circuit). But if we play a little bit, we will notice that there is a fundamental difference between the two cases: the LED in the Minus-circuit lights up when we move the vinyl plate toward the wire, while the LED in the Plus-circuit lights up when we move the plate away from the wire. If we bring the loose ends of the wires close to each other and swing the plate, then the LEDs light up alternately.
Now, if we take the glass, rub it with the silk cloth, we will notice that the reverse happens: the LED in the Plus-circuit lights up when we move the glass toward the wire, while the LED in the Minus-circuit lights up when we move it away from the wire. If we don’t move the electrified objects, absolutely nothing happens, no matter how close they are to the wires. As mentioned before, this is quite feasible with only one transistor per circuit, yet the movements of the vinyl plate and the glass have to be much more energetic. But even in this experiment with two transistors per circuit we can notice that the faster we move the electrified objects, the stronger the lamps light up.
The cloths after the rubbing produce the reverse effect from the rubbed objects. Still, their effect dies out much faster than that of the vinyl plate and the glass.
From this it becomes clear that vinyl and glass act completely opposite: vinyl actuates the minus-transistor (PNP) by moving toward, while glass by moving away from the wire end; vinyl actuates the plus transistor (NPN) by moving away from the wire end, whereas glass by moving toward it. We see that there are four cases:
Let’s make the next experiment with these circuits. Look please at the diagram below:
( Here I have drawn the transistors with their electronic symbols. )
The circuits are the same as before, with only one difference. Both long wires are in this variant connected either to the emitters or to the collectors of the transistors as shown above, but not to the both at the same time. That’s why one of the two variants is marked with dashed lines. If we now repeat the already described procedure with the electrified glass and the electrified vinyl, then absolutely nothing happens, that is, the LEDs don’t light up at all.
Pay attention now, please. In the following circuits, a piece of wire (15–20cm or longer) is connected to each of the bases as shown below (the pieces are colored in red).
The other ends of the red wires are loose. These ends are far from the loose ends of the other wires where we exert the action of the electrified objects.
So, if we now repeat the procedure with the electrified glass and the electrified vinyl, then the LEDs light up again.
But look, now it is all reversed. When we were moving the vinyl plate toward the loose ends of the wires of the first diagram, then the LED in the Minus-circuit lit up; while upon moving the plate away from them, the LED in the Plus-circuit lit up. Now it is upside down. The LED in the Plus-circuit lights up upon moving the vinyl plate toward the wires, while the LED in the Minus-circuit lights up upon moving the vinyl plate away from the wires. But without those wire pieces of 15–20 cm it was not possible.
All this is valid also in the case when we connect the long wire to the collector(s) instead of to the emitter.
Still, I will not give the explanation now, but will do it in my next post in this article.
These are extremely important experiments not only for a true understanding of the semiconductors, but also for a true understanding of the electric current in general. In those long wires it is flowing an electric current during the motion of the electrified objects.
If you read the articles below, maybe you will find the solution by yourself:
What is "ground" in electricity?
What is electromagnetic induction?
Electricity flows in an open circuit, too!
Is positive and negative electricity nomenclature arbitrary?
P.S. When the wires are pretty long, then it may happen that the LEDs shine dimly even without any exertion of the electric force of the glass and vinyl. In my view, it is happening because of the great electromagnetic smog in the air nowadays. The long wires in this case are actually antennas and since the two transistors offer a great amplification, the LEDs are shinning dimly.
To avoid this, increase the resistance in the circuits to 2 Kilo- ohms.
Why do I insist on long wires whose loose ends are far from the circuits themselves? Only to show that the exertion of the electric force of the electrified vinyl and glass has influence only on the wires and not at all on some elements of the circuits directly.
I urge every student and every teacher, who really seeks a true understanding of electricity, to replicate these experiments. It is one thing to read about an experiment, but completely other thing to sense it by oneself. It is like to read about what it looks like to live on Antarctica and to spend some time there personally.
