Take a good look at the Dynon IM page. It describes how to use the shunt, but not exactly what it does or why it works.
So, fluid systems are often used to describe how electricity works in circuits. They're very different, but they can illustrate fundamental principles without getting lost in the math weeds. (see
huntaero's comment). Now the details.
Think about a spigot with two garden hose connections and a well pump that supplies 50 psi. We'll call that voltage for electricity.
Then think about connecting two hoses to that spigot, one 2" in diameter, and one 2 mm in diameter (about 1/4").
Turn on the faucet and open the valve for the 2" hose (50 mm). Water will flow. We'll call that current for electricity.
Take a good picture of a typical garden hose in your yard (or at the airport when you're washing the plane).
You will see lots of water flowing through the open end of the 2" hose but not a huge amount of pressure, unless you put your thumb over the end of the hose. Then you'll feel pressure on your thumb and the water will spray out faster and harder. We'll call your thumb resistance.
That is the fundamental way electricity works: a driving force called voltage, a flow of electrons (or charges in this case negative) called current measured in amperes. Amps are simply a measure of how many charges are flowing in a wire over a fixed time, and that flow is governed by how much resistance is in the wire or circuit components.
Now if you bend your flowing garden hose over, you've created a blockage turning off the flow. Once the pressure in the hose reaches the pressure from the pump it stops flowing. Or you can turn off the valve and get the same thing and the water will stop flowing a few seconds after you turn off the valve, blocking the flow, provided it is strong enough to stop the water.
Now, turn on the valve for the very small hose. What do you get? A powerful stream of water. The small hose offers a major difference in resistance to flow compared to the big hose.
So it is with a shunt. There is a big wire (that flat metal plate between the binding posts) which nearly all of the current flows powering the ship's electrical system or whatever is downstream of it as the Dynon diagram shows. Very low resistance, large current flow, little voltage (pressure) drop.
But the "shunt" portion is the very small hose, very high resistance, small current flow.
So, when you open both valves on the spigot and let water flow through both hoses at the same time, which one will carry more water (current)?
The big one, of course, which is why we use big hoses to carry a lot of water. But the trickle of water coming out of the small hose is still useful for figuring out the flow through the large hose. Same amount of water passing through both hoses, but vastly different flow rates.
The voltage difference (pressure difference) across a high resistance will be smaller for the same current flowing from the battery, than it will be across the low resistance. (Ohms law combined with Kirchoff's Current Law).
This diagram shows the relative currents across the shunt. R1 is the small hose, R2 is the big hose. Current coming in (IT) = current going out.
Current crossing the high resistance (R1) is different from low resistance (R2).
So, this brings the obvious question: Why do we need a shunt at all? Why not just stick a meter in the feed line from the alternator and be done with it?
And the reason is that most aircraft alternators can make about 60 to 140 Amps of current. To meter these circuits directly you need a winding in the meter with wire big enough to handle the full current load. Take a look at the wire coming off the battery. Big heavy meter.
With the shunt we only need to pass a few milliAmps and measure current across that high resistance so the meter can use very fine wire to measure great voltage swings without burning out. All voltmeters work by creating a current flow through a coil which creates a magnetic field and swings the needle. (digital meters have similar principles but different implementations).
Hope this is helpful to the airline pilots out there. Especially those with grandchildren like mine who when he was four, sitting in the right seat gave me the FO roll eyes look that says he's thinking, "Does the captain know what the heck he's doing!? or is he out of his mind????"
He gives me the same look when I tell him we'll talk about Heisenberg's Uncertainty Principle tomorrow.
