Four long straight wires pass through the corners A, B, C and D of a square, perpendicular to the page, as in the figure. Each carries the same current $I$: out of the page in A and B, into the page in C and D. A fifth long straight wire passes through the centre O of the square and carries a current $I$ out of the page. Only the four other wires act on the fifth. In which direction is the force on the fifth wire?

Fig. 1: End view of five long straight parallel wires that are perpendicular to the page. Four of them are at the corners of a square: A at the top left and B at the top right, each drawn with a dot for a current out of the page, and C at the bottom right and D at the bottom left, each drawn with a cross for a current into the page. All four carry the same current I. The fifth wire passes through the centre O of the square and is drawn with a dot, so its current I also comes out of the page.