If you press the pushbutton switch, you realize that the LED1 lights up, albeit weakly. This indicates a current now flows through the LED1 and this time the current can only flowing through the transistor.
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This current has as a consequence that the transistor switches through and a much larger current flows from the collector to the emitter. The LED2 lights up bright! 
Circuit with Transistor. (Enlarge) |
Circuit diagram with transistor. |
Currents through a switched through NPN transistor. |
The illustration shows the currents I, which flows in a switched through NPN transistor.
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