I = neAvd: microscopic current formula; vd ~10^{-4} m/s; current is instantaneous because E propagates at ~c.

Resistance: R = ρl/A; metals α > 0 (R↑ with T); semiconductors α < 0 (R↓ with T).

Power trap: P_parallel = $n^{2}$ × P_series for n identical resistors on the same battery. Use $I^{2}$R for series; $V^{2}$/R for parallel.

Terminal voltage: V = ε − Ir (discharge); at null point on potentiometer I = 0, so V = ε (true EMF).

Null methods: Wheatstone P/Q = R/S; Metre bridge R/S = l/(100−l); Potentiometer ε_{1}/ε_{2} = l_{1}/l_{2}; r = R(l_{1}−l_{2})/l_{2}.