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So I was reading about oscilloscopes and an article described their function as "displaying a graph of voltage over time."

I had a strange sense of disappointment. What about resistance? What about capacitance? What about current? Obviously my multi-meter can do those but surely an oscilloscope - the Daddy of all test equipment - can do everything?!

Then it suddenly made me wonder whether it's because at the end of the day voltage is the most important thing going on in a circuit. And that led me to wonder what actually constitutes a "signal" in a circuit - particularly an audio circuit.

So my question is, is it only voltage that can be considered a "signal"?
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I guess a "Signal" could also be in the RF range or microwave, or even in the light range.

So perhaps a "Signal" can be considered any form of modulation of a component of the electro magnetic spectrum.

But then I am always open to correction, only a truly ignorant person believes that they are not ignorant in some way.

Cheers Pete.
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You can learn a lot about what is going on in a circuit by monitoring voltage over time.  Thanks to ohms law and the like.

http://hades.mech.northwestern.edu/index.php/Resistors_%28Ohm%27s_Law%29,_Capacitors,_and_Inductors
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There are only two variables in any electrical circuit, the primary (or independent) one is voltage, and the secondary (or dependent) one is current.  It is voltage which drives the current through whatever passive device is attached to the circuit, whether it be resistance, capacitance or inductance.  Again it is voltage which excites active devices (semiconductors etc), whether they be voltage or current driven.

Hence by measuring voltage (on an oscilloscope) with respect to time you can establish or analyse what is happening within any circuit.
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There are only two variables in any electrical circuit, the primary (or independent) one is voltage, and the secondary (or dependent) one is current.  It is voltage which drives the current through whatever passive device is attached to the circuit, whether it be resistance, capacitance or inductance.  Again it is voltage which excites active devices (semiconductors etc), whether they be voltage or current driven.

Thank you, that's helpful. In what sense might a device be voltage or current driven? I can imagine how something might be voltage driven - anything in fact. But what would be an example of something current driven?
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An LED is current driven. 
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I don't think you connected the grounds, Dave.
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But what would be an example of something current driven?
A ordinary NPN or PNP is current driven, but a FET is voltage driven.
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But what would be an example of something current driven?
A ordinary NPN or PNP is current driven, but a FET is voltage driven.

Ok so ultimately it's on a component by component basis dependent upon it's physical characteristics/theory/engineering Etc and is something you learn as you go along studying different components/techniques and areas of electronics?
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