A thermo-magnetic actuator comprising: a housing (3, 4); a chamber within the housing; a working fluid changing phase upon heating and located in the chamber; a rolling diaphragm driven by the working fluid; a piston contacting and driven by the rolling diaphragm and sliding within the housing (3, 4) upon expansion of the working fluid; an electrical heater disposed within the chamber for heating the working fluid upon application of electrical power to the electrical heater; a piston rod slidably disposed in the housing (3, 4) and movable between a retracted position, retracted with respect to the housing (3, 4), and an extended position extending outward of the housing (3, 4), the piston rod being driven and sliding from the retracted position to the extended position in response to heating of the working fluid by the electrical heater; a first spring biasing the piston rod toward the retracted position; and an electromagnet disposed on the housing (3, 4) and generating a magnetic field when an electric current is supplied to the electromagnet, the thermo-magnetic actuator being characterized by: the electromagnet being disposed at an end of the housing (3, 4), a ferromagnetic target assembly (23, 30) attached to the piston rod, sliding within the housing (3, 4), and attracted toward the electromagnet, when an electrical current flows through the electromagnet, to maintain the piston rod in the extended position, even if electrical power is no longer applied to the heater, wherein the ferromagnetic target assembly (23, 30) includes a ferromagnetic core to which one end of the piston rod is attached, the ferromagnetic core sliding within the housing (3, 4), and a guide pin attached to the ferromagnetic core and extending from an end of the ferromagnetic core opposite the end of the ferromagnetic core from which the piston rod extends, the piston including a hollow portion receiving the guide pin, and a second spring biasing the ferromagnetic target assembly (23, 30) away from the piston, wherein the second spring is a coil spring, which surrounds the guide pin and is interposed between the ferromagnetic core and the piston. a housing (3, 4); a chamber within the housing; a working fluid changing phase upon heating and located in the chamber; a rolling diaphragm driven by the working fluid; a piston contacting and driven by the rolling diaphragm and sliding within the housing (3, 4) upon expansion of the working fluid; an electrical heater disposed within the chamber for heating the working fluid upon application of electrical power to the electrical heater; a piston rod slidably disposed in the housing (3, 4) and movable between a retracted position, retracted with respect to the housing (3, 4), and an extended position extending outward of the housing (3, 4), the piston rod being driven and sliding from the retracted position to the extended position in response to heating of the working fluid by the electrical heater; a first spring biasing the piston rod toward the retracted position; and an electromagnet disposed on the housing (3, 4) and generating a magnetic field when an electric current is supplied to the electromagnet, the thermo-magnetic actuator being characterized by: the electromagnet being disposed at an end of the housing (3, 4), a ferromagnetic target assembly (23, 30) attached to the piston rod, sliding within the housing (3, 4), and attracted toward the electromagnet, when an electrical current flows through the electromagnet, to maintain the piston rod in the extended position, even if electrical power is no longer applied to the heater, wherein the ferromagnetic target assembly (23, 30) includes a ferromagnetic core to which one end of the piston rod is attached, the ferromagnetic core sliding within the housing (3, 4), and a guide pin attached to the ferromagnetic core and extending from an end of the ferromagnetic core opposite the end of the ferromagnetic core from which the piston rod extends, the piston including a hollow portion receiving the guide pin, and a second spring biasing the ferromagnetic target assembly (23, 30) away from the piston, wherein the second spring is a coil spring, which surrounds the guide pin and is interposed between the ferromagnetic core and the piston. the electromagnet being disposed at an end of the housing (3, 4), a ferromagnetic target assembly (23, 30) attached to the piston rod, sliding within the housing (3, 4), and attracted toward the electromagnet, when an electrical current flows through the electromagnet, to maintain the piston rod in the extended position, even if electrical power is no longer applied to the heater, wherein the ferromagnetic target assembly (23, 30) includes a ferromagnetic core to which one end of the piston rod is attached, the ferromagnetic core sliding within the housing (3, 4), and a guide pin attached to the ferromagnetic core and extending from an end of the ferromagnetic core opposite the end of the ferromagnetic core from which the piston rod extends, the piston including a hollow portion receiving the guide pin, and a second spring biasing the ferromagnetic target assembly (23, 30) away from the piston, wherein the second spring is a coil spring, which surrounds the guide pin and is interposed between the ferromagnetic core and the piston.