A circle is drawn on the plane. How to use only a ruler to draw a perpendicular from a given point outside the circle to a given line passing through the center of this circle?
1. **Identify the given elements:**
   - Let the given point outside the circle be \( A \).
   - Let the given line passing through the center of the circle be \( l \).
   - Let the center of the circle be \( O \).

2. **Draw the diameter of the circle:**
   - Use the ruler to draw the diameter \( BC \) of the circle such that \( O \) is the midpoint of \( BC \).

3. **Construct triangle \( ABC \):**
   - Connect point \( A \) to points \( B \) and \( C \) using the ruler to form triangle \( ABC \).

4. **Mark intersections with the circle:**
   - Extend line \( AB \) to intersect the circle at point \( E \).
   - Extend line \( AC \) to intersect the circle at point \( F \).

5. **Identify special cases:**
   - If \( AB \) is tangent to the circle, then \( B = E \).
   - If \( AC \) is tangent to the circle, then \( C = F \).

6. **Use the properties of the circle and triangle:**
   - By the inscribed angle theorem, \( \angle AEB \) and \( \angle AFC \) are right angles because \( E \) and \( F \) lie on the circle and \( B \) and \( C \) are endpoints of the diameter.

7. **Identify the orthocenter:**
   - Since \( BF \) and \( CE \) are perpendicular to the sides of triangle \( ABC \), their intersection point \( H \) is the orthocenter of \( \Delta ABC \).

8. **Draw the altitude:**
   - Draw line \( AH \) using the ruler. Since \( H \) is the orthocenter, \( AH \) is an altitude of \( \Delta ABC \).

9. **Conclusion:**
   - Since \( AH \) is an altitude, it is perpendicular to \( BC \). Therefore, \( AH \) is the required perpendicular from point \( A \) to the line \( l \) passing through the center of the circle.

The final answer is \( \boxed{ AH } \) is perpendicular to \( BC \).