I found this explanation of a shut that helped me understand how the ammeter works
https://www.bluesea.com/support/arti...o_a_DC_Ammeter
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How a Shunt Works. It is useful to think of water flow in a pipe when thinking about current flow in a wire. The way that a shunt works is analogous to a restriction to the flow of water in a pipe and a bypass around the restriction. Some of the water flow in the Main Pipe is diverted through the Bypass. A Paddle Wheel in the Bypass measures the water flow through the Bypass. If the Bypass allows 1/100th of the water in the main pipe to flow through it, and the Restriction allows the remaining 99/100ths of the water to flow through it, the ratio of flow in the Bypass to the Restriction is 1 to 99. Using this ratio, the Paddle Wheel could be calibrated to indicate the total flow through the Main Pipe.
A DC ammeter and shunt works in a similar way—a small amount of current that flows through the Main Wire is diverted to, and measured by, the Meter. Analog meters have very fine internal wires that flex to enable the needle to move. Because the wires are fine, they carry only a very small current. Therefore, the current in the meter must be a tiny fraction of the total current to be measured.
In order to obtain an accurate reading of the current flow through the main wire, the shunt and meter are very precisely calibrated at fixed resistance values—the meter resistance is typically 50 Ohms and the shunt resistance is a fraction of an Ohm.
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