Four units run this whole chapter, and a water pipe explains them all. Current $I$, in amperes, is the flow — how much charge streams past per second. Charge $Q$, in coulombs, is the water itself: one coulomb is what one ampere delivers in one second. Voltage $V$, in volts, is the pressure pushing the flow along. And resistance $R$, in ohms (the symbol is $\Omega$, said omega), is the narrowness of the pipe fighting back.

The first machine is the counting one: $Q = I t$ (read aloud: Q equals I t) — a steady current held for a time moves a definite amount of charge. Amperes times seconds IS coulombs; the units already agree, no bridge required.