Which statement about detecting the end position of a piston rod is true?

The statement that the field of a magnet causes the two reeds to close is true because it accurately describes the functioning of reed switches, which are commonly used in piston rod position detection systems. In these systems, a magnet attached to the piston rod activates the reed switches. When the magnet approaches the reeds, they respond by closing, creating a circuit that can signal the end position of the rod. This method is reliable, as it allows for non-contact detection, which minimizes wear and tear compared to visual observation or pressure change methods.

Using reed switches thus implements a straightforward and efficient method for confirming whether the piston rod has reached its intended end position. The other options either misrepresent how reed switches operate or suggest methods (like visual observation or pressure changes) that are less effective compared to magnetic detection for this specific application.