Reed switches operation app note from Hermetic Switch Inc. (HSI). Link here (PDF)
A reed switch is activated by a magnetic !eld. It’s important to realize that there are numerous possibilities for a switch’s orientation within a given magnetic field.
The length of the magnet and the length of the reed switch both affect the magnetic field coupling. Removal of the switch lead material reduces the “magnetic antennae.” The magnetic coupling is reduced, the sensitivity is decreased, and the magnet must get closer to the reed switch to activate it. This is a general rule for a parallel mode of operation