Do two currents push on each other?

That time, that place

In April 1820 Ørsted noticed it mid-lecture in Copenhagen: a compass needle beside a live wire swung aside.

The news reached Paris that September. Ampère began experimenting that week and reported the following week.

Why this question

His question: if a current moves a magnet, do two currents push on each other?

He laid two wires side by side and passed current through both. Same direction, they attract; opposite, they repel.

Then what is a magnet? Ampère proposed that a magnet is magnetic because tiny currents circulate inside it.

What was found

The guess was right. We now know a magnet's magnetism comes from the spin of electrons. He got the structure right without knowing atoms existed.

On this joining of current and magnetism Faraday stood, and on Faraday, Maxwell.

It is why the unit of current is the ampere — the number printed on the socket.