An ophthalmic surgical apparatus comprising: a support base; a tower extending upward from the support base; a user-positionable arm extending from the tower and having a supported end that is attached to the tower; and an optical instrument attached to a distal end of the user-positionable arm, the user-positionable arm comprising: a four member linkage comprising four members (132, 134, 136, 138) that are pivotably connected to each other; an electromagnetic brake strut (150, 250) pivotably attached in a diagonal relationship to at least two of the four members at a first strut pivot joint and at a second strut pivot joint, the electromagnetic brake strut comprising: a rotatable lead screw comprising: a first lead screw end region; and a second lead screw end region; a nut attached to the first strut pivot joint, the first lead screw end region received into the nut; characterised in that the electromagnetic brake strut further comprises an electromagnetic clutch attached to the second strut pivot joint, wherein the electromagnetic clutch comprises: a rotor coupled to the lead screw such that the rotor and the lead screw rotate together; a friction plate slideably received into the rotor; a brake housing comprising a friction engaging surface configured to engage a surface of the friction plate; and a spring disposed between the rotor and the friction plate, the spring configured to bias the friction plate towards the brake housing, and wherein the brake housing further comprises: a electromagnet; and a magnet operable to produce a first magnetic force that attracts the friction plate such that the brake strut (150,250) is in a locked configuration, when the electromagnet is de-energized; and the electromagnet operable to produce a second magnetic force that exceeds the first magnetic force that repels the friction plate from the the brake housing when the electromagnet is energized, such that the electromagnetic clutch is operable to permit rotation of the rotatable lead screw when an electric voltage is applied to the electromagnetic clutch. a support base; a tower extending upward from the support base; a user-positionable arm extending from the tower and having a supported end that is attached to the tower; and an optical instrument attached to a distal end of the user-positionable arm, the user-positionable arm comprising: a four member linkage comprising four members (132, 134, 136, 138) that are pivotably connected to each other; an electromagnetic brake strut (150, 250) pivotably attached in a diagonal relationship to at least two of the four members at a first strut pivot joint and at a second strut pivot joint, the electromagnetic brake strut comprising: a rotatable lead screw comprising: a first lead screw end region; and a second lead screw end region; a nut attached to the first strut pivot joint, the first lead screw end region received into the nut; characterised in that the electromagnetic brake strut further comprises an electromagnetic clutch attached to the second strut pivot joint, a four member linkage comprising four members (132, 134, 136, 138) that are pivotably connected to each other; an electromagnetic brake strut (150, 250) pivotably attached in a diagonal relationship to at least two of the four members at a first strut pivot joint and at a second strut pivot joint, the electromagnetic brake strut comprising: a rotatable lead screw comprising: a first lead screw end region; and a second lead screw end region; a nut attached to the first strut pivot joint, the first lead screw end region received into the nut; characterised in that the electromagnetic brake strut further comprises an electromagnetic clutch attached to the second strut pivot joint, a first lead screw end region; and a second lead screw end region; a nut attached to the first strut pivot joint, the first lead screw end region received into the nut; characterised in that the electromagnetic brake strut further comprises an electromagnetic clutch attached to the second strut pivot joint, wherein the electromagnetic clutch comprises: a rotor coupled to the lead screw such that the rotor and the lead screw rotate together; a friction plate slideably received into the rotor; a brake housing comprising a friction engaging surface configured to engage a surface of the friction plate; and a spring disposed between the rotor and the friction plate, the spring configured to bias the friction plate towards the brake housing, and a rotor coupled to the lead screw such that the rotor and the lead screw rotate together; a friction plate slideably received into the rotor; a brake housing comprising a friction engaging surface configured to engage a surface of the friction plate; and a spring disposed between the rotor and the friction plate, the spring configured to bias the friction plate towards the brake housing, and wherein the brake housing further comprises: a electromagnet; and a magnet operable to produce a first magnetic force that attracts the friction plate such that the brake strut (150,250) is in a locked configuration, when the electromagnet is de-energized; and the electromagnet operable to produce a second magnetic force that exceeds the first magnetic force that repels the friction plate from the the brake housing when the electromagnet is energized, such that the electromagnetic clutch is operable to permit rotation of the rotatable lead screw when an electric voltage is applied to the electromagnetic clutch.