FINGER EXPERIMENT: Dry finger resistance should be around 100k ohms. A wet finger should be around 1k ohms. The resistor on the left simulates the resistance of the finger you apply to the leads. A dry finger with 100k ohms will let a little current flow through to the Base of the NPN transistor and the current flowing from the Collector to the Emitter of the NPN will be small, therefore the LED will barely light-up. Change the value of the same resistor to 1k ohms and the LED lights-up with more intensity. A wet finger offers less resistance, more current flows to the Base of the NPN transistor and therefore the current flowing from the Collector to the Emitter of the NPN is higher and the LED shines more brightly. Conclusion: The small amount of current through the Base of the NPN controls how much current flows into the Collector and how much flows out through the Emitter, thereby controlling the brightness of the LED in the circuit. If you open the SPST switch in the top left section of the circuit, no more current will be applied to the Base of the NPN and therefore it will block the flow of electricity between the Collector and the Emitter.
