A transistor is an electronic switch that enables the current flow between two terminals if a voltage/current is present on a third.

Think of it as a push button switch. Push the button (apply current on the third pin) and current can flow between the other two. (very simplified)

For comparison:

A Capacitor stores electricity in the form of an electric field.

A resistor impedes the flow of electricity

An Inductor stores electricity in the form of a magnetic field.

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A Diode is an electronic component that only allows current to flow one direction across it; the most commonly known type of diode these days is an LED (Light Emitting Diode). There are many flashlights and even house lights that are made of LEDs and they are indeed diodes which only allow current to flow in one direction (and then also happen to emit light when that happens).

A resistor allows current to flow both directions but resists the flow of current. A Fuse is something that generally allows current to flow in either direction but when too much current (or at too high of a voltage) flows across it the fuse "blows" (is destroyed) and the flow of current stops. A driven element is the part of an antenna that a transmitter causes to emit RF energy.

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A transistor has three terminals; on the most common type of transistor (a bipolar transistor) these are Base, Collector, and Emitter. Current applied to the Base controls current flow between the collector and emitter, which allows it to work as an electronic switch. The amplifier capabilities are a little more complicated and have to do with the fact that the amount of current directed into the Base is directly proportional to the amount of current that can flow through the Collector and Emitter.

Conversely, an Oscillator is something that produces a signal (such as an audio tone for CW). A Potentiometer is a variable resistor, and a Voltmeter is a tool that can be used to measure voltage. None of these have the potential to be a switch of any kind.

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While the specifics of how a bipolar junction transistor works is beyond the scope of this explanation, it may be helpful to remember that nearly any time you hear the term "semiconductor" in casual electronic jargon someone is talking about either a transistor specifically or about something made with many transistors (such as an integrated circuit).

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http://en.wikipedia.org/wiki/Transistor

A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal.

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Electrical components can be small. Sometimes too small to be able to read letters or words. Might also think of a "striped cat"

Click for a list of schematic symbols.

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(L)ight

(E)mitting

(D)iode

http://en.wikipedia.org/wiki/Led

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A Field Effect Transistor controls voltage and current like a regular transistor, but can operate on a much smaller signal, which makes it ideal for radio receivers. It operates using an electric field to control the shape of the channel in the semi-conductor material.

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Electrodes are refered to as either Anodes or Cathodes regardless of whether they are on a diode or other component. Current flows into an Anode and out of a Cathode.

Plus and Minus indicate a positive or negative voltage on a power source but are not strictly speaking names for electrodes. Source, Gate and Drain are terminals on a Field Effect Transistor (FET). Base, Collector and Emitter are terminals on a bipolar transistor.

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Transistor is the only component in this list that has gain.

A transformer may have a high secondary voltage ratio and be used for increasing voltage, or it may have a high secondary current ratio and be used for increasing current, but these increases and decreases are called transformation or conversion not gain.

So the only option that makes sense in this list is Transistor.

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Gain is defined as being the ratio of the output power to the input power, or in other words it refers to how much power is gained when passing through the transistor. Just remember that "you gain a lot through amplification".

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