I'm currently trying to relearn all electronic principles from the ground up and I'm reading this book about it as well. I understand that in the water analogy voltage could be like water level/pressure, current the speed of the water and resistance the diameter of the pipe.

I'm getting a little bit confused about the relationships between these things though since a potentiometer changes resistance that would affect the current, right? Since if it's resisted the current decreases,

My question comes in when you measure the voltage between the middle terminal and another and then turn the potentiometer it ends up changing the voltage but wouldn't the voltage stay the same since the difference in pressure is the same, only the speed?

Also what exactly is current, like how does a component "know" how much current to draw, how does that work?

Honestly I'm really confused and any other definitions which could better define these properties would help. Thank you!

Edit: Added extra question about current.


2 Answers 2


By the water analogy, the water pressure at one end of a pipe must be different from the pressure at the other end, for there to be a flow of water through it.

With different pressures at each end, water pressure inside the pipe, at various points, must have some value between those extremes. Pressure doesn't stay high all the way until the other end, and then suddenly drop, it's a gradual decrease of water pressure as you "travel" along it from the high pressure end to low.

Your voltmeter, with one end connected the potentiometer wiper, is simply measuring the the difference in "pressure" between one end (of known, fixed pressure) and some arbitrary point along the pipe corresponding to wiper position.

Current is the same all the way along the resistive path, along the entire length of it, in the same way that if one litre per second enters one end of the pipe, then one litre must exit the other end each second (unless there's a leak, or the pipe is ballooning).

Don't fixate on speed, if you insist on using the water analogy, think in terms of litres per second of flow.

I prefer a road traffic analogy; picture two roadways, a single-lane one, and a three lane one. If I tell you that an observer counts 10 cars passing his position each minute, in both scenarios, then it's easy to see that cars in the single lane road must be travelling 3 times faster than cars in the three-lane road.

In these analogies, current is not speed, current is cars per minute, or litres per second. In electronics, current is charge per second passing some point along a conductor, so the analogy is not a bad one. An ammeter is like the roadside observer, counting cars that pass his position, and reporting how many do that each minute.

Let me extend the road traffic analogy a little. Let's treat voltage as people's motivation to travel. There's a new show in town, everybody want to see it, so they all travel into town, along the three lane highway. Let's change polarity. While they are watching the show, they get news that the town's about to get nuked. Now they all travel, much, much faster along that highway, in the other direction. Same people, different speed and direction.

While speed is obviously a factor, the only good measure of influx or evacuation is how many cars "arrive" per minute, which is only indirectly related to their speed.

And, while speed plays a role in that figure, motivation does too. Assuming no accidents, and an orderly migration of people, cars per minute is a function of both motivation and the road's width.

In this analogy, how does the road know how many cars to pass in a minute? It doesn't. Do the drivers all know how long the journey will take? No. Did the decision to evacuate hinge on any consideration of lane count or distance? No. What happens is simply that people go where they want, at a rate ultimately outside of their control, along paths which they can't change. The resulting relationship between traffic flow, lane count and motivation is determined by all three factors, simultaneously. No individual element in this system has any clue about any of the others, but the system settles into some equilibrium which we call Ohm's law.

  • 1
    \$\begingroup\$ That analogy was wonderful and it really helped understand, it sort of hard to imagine just an increase in diameter with water flowing and stuff but lanes of countable cars really clicked Thank you! \$\endgroup\$ Commented Jun 15, 2023 at 6:58
  • \$\begingroup\$ Now I am confused. I don't see why the cars on the single lane road must go 3 times faster than those on the three lane. Consider the case where all ten cars are staying close to each other, and only those cars are seen by the observer in one minute. If the roadway is 1 mile, then all the cars are going 60 MPH. It doesn't matter if the cars are three abreast, or all single file. \$\endgroup\$
    – PStechPaul
    Commented Jun 17, 2023 at 8:36
  • \$\begingroup\$ @PStechPaul I neglected to mention car density. I assume cars per mile is the same for all lanes in both cases. \$\endgroup\$ Commented Jun 17, 2023 at 9:01

OP's questions

... since a potentiometer changes resistance that would affect the current right? Since if it's resisted the current decreases...

... when you measure the voltage between the middle terminal and another and then turn the potentiometer it ends up changing the voltage but wouldn't the voltage stay the same...

To answer these OP's questions, we obviously need to clarify what a "potentiometer" and "rheostat" are. I will do this in several steps.

Controlling current

In the most elementary (Ohm's) circuit (see the schematic below), a voltage source V produces current I that is "proportionally limited" by the resistance R (I = V/R). We can change the current either by changing the voltage or resistance (in the latter case, we should be careful that the resistance does not become zero but, for example, > 100 Ω).

Variable resistor

If we decide to change the resistance somehow (here by the CircuitLab DC sweep simulation)...


simulate this circuit – Schematic created using CircuitLab

... we see that the dependence of current on resistance is non-linear because R is in the denominator of Ohm's law.



Let's see how a variable resistor was made in one of the first electrical inventions of the 19th century - the so-called "rheostat". Interestingly, the (total) resistance does not actually change (it is the maximum); only the place on the resistance wire where we connected through the wiper changes.


simulate this circuit

Let's now investigate the rheostat by the DC sweep simulation by sweeping its wiper position (K). As above, we are careful that the resistance does not become zero (we choose K => 0.1). We see the same non-linear curve as above.


Controlling both current and voltage

Rheostats used alone are not particularly useful; so let's add another but constant resistor R in series.


simulate this circuit

Now, when we change the rheostat's resistance, both the current through...

STEP 2_1

... and voltage across the load change.

STEP 2_2

Today, this configuration is used in simple transistor amplifier stages where the transistor collector-emitter junction is connected in series to a constant resistor Rc.

Controlling voltage

But what if we want to change only the voltage without changing the current? We need this when, for example, there is no load connected (open circuit). Hmm... Let's think about it...

The input voltage is constant. It is applied (see the schematic above) to a circuit of two resistors - P and R, in series. So, in order for the current not to change, the total resistance P + R should not change. But P changes; therefore R must also change but in the opposite direction!

Connecting two rheostats

This means to replace the constant resistor R with another rheostat P2 (see the schematic below) and to move its wiper in the same way as P1. Quite inconvenient... although all complementary ("push-pull") transistor stages work this way today.


simulate this circuit

"Inventing" the potentiometer

Unbelievable but fact - two centuries ago they thought of utilizing the third (unused until then) terminal of the rheostat and thus made one of the most ingenious inventions of the 19th century - the potentiometer.


simulate this circuit

Now when we move the wiper, one half increases its resistance and the other decreases it by the same amount. As a result:

  • the total resistance remains constant
  • since the input voltage is constant, the current is also constant
  • the transfer characteristic is linear


Bipolar-supplied potentiometer

The circuit can become quite beautiful and symmetrical if supplied bipolar...


simulate this circuit

... its curve also.



Usually, the rheostat and potentiometer are the same device - a constant resistor with three terminals, which is used in a different way:

  • The rheostat regulates the current in the circuit and possibly the voltage across the load (if the latter has any resistance); so the voltage, resistance and current change.
  • The potentiometer does not actually adjust anything because the total voltage, resistance and current do not change; it only changes the point at which the voltage is measured.
  • \$\begingroup\$ Downvoter, what didn't you like in my story about the humble potentiometer? \$\endgroup\$ Commented Jun 17, 2023 at 8:08
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    \$\begingroup\$ Wasn't me, but perhaps someone was inconvenienced reading all those words without the TL/DR warning? \$\endgroup\$
    – PStechPaul
    Commented Jun 17, 2023 at 8:53
  • \$\begingroup\$ @PStechPaul, Maybe... if the number of words is more important to them than what they say... \$\endgroup\$ Commented Jun 17, 2023 at 9:02

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