# Who changed $i$ to $j$ in electronics?

In electronics, $$j$$ is used for a square root of $$-1$$, because $$I$$ is current. Who introduced this and when? And was it really necessary, given that (at least now) current's symbol is capitalised? Perhaps it wasn't at the time, but as best I can tell Ampère didn't use lower case.

• Current in electronics certainly is represented by a small italic i today, particularly for small signal analysis, where complex arithmetic is used extensively! But as for when that started, I couldnt tell you. – Sam Gallagher May 27 '19 at 12:20
• @SamGallagher I was only ever taught to use $I$, but I can understand why a field called "small signal analysis" would use $i$, because it's probably focused on perturbations to background electromagnetism. – J.G. May 27 '19 at 12:51
• Its a simplified model for circuit analysis, where changes are assumed very small in active (e.g. transistor, op amp, etc) circuits. All "small signal" voltages and currents are represented by lower-case v and i. – Sam Gallagher May 28 '19 at 13:11
• Note that i refers to $\sqrt{-1}$ while i is often the unit vector in the x-axis. – Carl Witthoft May 28 '19 at 13:30
• @CarlWitthoft: The unit vector would normally be $\hat{\textbf{i}}$, not just $i$. – Ben Crowell May 30 '19 at 0:17

The symbol i is still used in mathematics; j is an invention of an electrical engineers, usually attributed to Charles P. Steinmetz. See for example Introduction of $\imath$ and $\jmath$ notations for the imaginary unit 