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.
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
As to why j was chosen, the following is quoted from a 1921 book, "Tables of Complex Hyperbolic and Circular Functions" by Arthur Edwin Kennelly. I am sure older ones will also explain the same.
My feeling is that since j was chosen by a German-born American (Steinmetz), it may be possible to attribute the similarity of i and j in German e.g. Iodine (symbol I) is Jod (symbol J) in German. However the letter i was also introduced by Gauss, who happened to be German, but at that time, Latin was the language of science. This is just my pure speculation.