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The stop action magnet, usually abbreviated to SAM, is an electromagnetic device used for the control of pipe organs and virtual pipe organs, and forms part of the organ's combination action. On a classical organ the device may be referred to as a drawstop solenoid.The SAM can be considered an electrical relay, the difference being that the SAM also has a drawknob or a tab, which enables it to be operated manually as well as electrically.
A SAM will have an armature which is operated by an electrically induced magnetic field and dependent on whether the SAM is being used with a drawknob or a tilting tablet on a classically voiced organ or with a flat tab on a theatre organ, the armature will move in either a linear fashion or with semi-circular movement, in a small defined arc. A drawknob or tab is attached to the armature to facilitate hand operation.
Each SAM consists of two coils of wire, wound on bobbins, which surround a single central, magnetically conductive pole piece or two separate pole pieces and will have one or more switching contacts or reed switches that are switched on or off by the movement of the armature. The switching contact is used to control which rank or ranks of pipes will sound when keys on a keyboard are depressed, although the actual 'rank switching' is often handled by a more complex electromagnetic or electronic relay system.
In mechanical operation using a SAM with a drawknob attached, a player will either pull the drawknob towards themselves to select an 'on' state, or push the drawknob away to select an 'off' state. The movement of the drawknob affects the state of the switch or reed contact. Similarly, with a SAM that utilises a tilting tablet, the player will tilt the tablet up or down by hand to operate it. With a theatre organ the tab attached to each SAM is raised or lowered by the player to operate the switch contact/s.