How to make a perpendicular?
How to make an perpendicular bisector?

1. Place your compass point on P and swing an arc of any size below the line that crosses the line twice. You will be drawing at least a semicircle. (Note: While you can draw this arc above or below the line, below the arc keeps the construction lines from bumping into one another.)
2. Stretch the compass LARGER!
3. Place the compass point where the arc crossed the line on one side and make a small arc above the line (the arc could be below the line if you prefer).
4. Without changing the span on the compass, place the compass point where the first arc crossed the line on the OTHER side and make another arc. Your two small arcs should be intersecting.
5. Using a straightedge, connect the intersection of the two small arcs to point P.

Regards