0A transistor can be used as a switch and signal amplifier. It is an electronic device with three contacts: the emitter (E), base (B) and collector (C). A very small current on the transistor's base can control a much larger current flowing through a passage between collector and emitter. The following drawing shows the concept behind an NPN transistor using a water analogy (I found it in my old Kosmos "Electronic Junior" book from Germany). If there is no water flowing down the base channel, the gate between the collector and the emitter channel is closed, no water can flow from the collector to the emitter. If there is water flowing down the base channel it lifts the gate that normally blocks the collector/emitter channel. Once this gate is open, water flows from the collector to the emitter.
