Magiplug® is a bath plug which opens to let water out when the head of water over it reaches a certain level, intended to prevent bath overflow.
The design has a significant advantage over conventional (built-in) overflows in that the emergency outflow takes place under a very much greater head and thus at a very much greater flow rate.
The Magiplug will be able to "keep up" with continuing inflow to a greater degree than a conventional overflow. Magiplug® is constructed from moulded plastic parts.
The seal is maintained by a spring. Compression of this spring under increasing head of water over the plug eventually causes the plug to open, allowing water outflow. At some lower level, the spring will restore a closed position and will shut off outflow.
The plastic forming the open / close sealing plate has been modified by the addition of a temperature-sensitive colouring.
This aspect of design is to give an immediate visual indication of potentially over-hot bath water.
When the pressure plate turns green this can be taken to indicate water definitely not too hot for safe exposure levels for those with fragile skin or reduced sensation.