a current divider
A current divider is the partner idea to the voltage divider. When a current arrives at a junction where two resistors sit in parallel, the current splits between them, with more of it taking the easier, lower-resistance path. Picture a stream reaching a fork with a wide channel and a narrow one: most of the water flows down the wide channel, but some still trickles down the narrow one. The two branch currents always add back up to the current that arrived.
For two resistors in parallel, the current in branch 1 is I1 = I_total times R2 / (R1 + R2). Watch the twist carefully: the resistor that appears on top is the other branch's resistor, R2, not R1. That is because the branch with the smaller resistance must grab the larger share. For example, if 9 mA arrives at a 1 kohm in parallel with a 2 kohm, the 1 kohm branch carries I1 = 9 times 2 / (1 + 2) = 6 mA, and the 2 kohm branch carries the remaining 3 mA.
Current dividers explain how a shunt resistor lets you measure a large current by diverting a known fraction, and how current shares between parallel paths on a circuit board or inside a chip. The common slip to avoid is forgetting the cross: people instinctively put R1 on top for I1 and get the split exactly backwards, sending the bigger current down the more resistive path, which cannot happen.
A 100 A motor current is too big to meter directly, so it passes through a 0.001 ohm shunt, and the meter taps a tiny parallel branch. The current-divider ratio sets exactly what fraction the delicate meter sees, keeping it safe.
Current divides in favour of the lower-resistance path; the formula puts the OTHER branch's resistor on top.
The branch with the smaller resistance carries the larger current, and the two-resistor formula puts the opposite branch's resistor in the numerator. Getting the cross backwards is the usual error.