An apparatus comprising: means for transferring momentum from a first mass element (32, 32') to a second mass element (34, 34') located within the first mass element so that kinetic energy is transferred from the first mass element to the second mass element through sequential collisions, the first mass element having a mass larger than the second mass element, wherein either the first mass element includes one or more permanent magnets and the second mass element includes one or more coils or the second mass element includes one or more permanent magnets and first mass element includes one or more coils, wherein the second mass element is mounted within the first mass element such that the second mass element is free to move within the first mass element and wherein the first mass element is connected to a top of the apparatus via a first spring and/or to a base of the apparatus via a second spring; means for at least partially locking the second mass element to a housing of the apparatus when the first mass element and/or the second mass element has an acceleration below a first threshold, wherein the means for at least partially locking the second mass element to the housing comprises a magnetic locking arrangement; and means for inducing a current in the coils when the second mass element moves relative to the first mass element, wherein: the means for at least partially locking the second mass element to the housing is configured to restrict or prevent transfer of momentum from the first mass element to the second mass element and/or to restrict or prevent relative motion between the first mass element and the second mass element when the first mass element and/or the second mass element has an acceleration below the first threshold, wherein an attractive force of the magnetic locking arrangement is overcome when the acceleration is above the first threshold such that an inertial force on the second mass element overcomes the attractive magnetic force, allowing the second mass element to move freely when impacted by the first mass element; and in use, the first and second mass elements and the housing of the apparatus remain aligned. means for transferring momentum from a first mass element (32, 32') to a second mass element (34, 34') located within the first mass element so that kinetic energy is transferred from the first mass element to the second mass element through sequential collisions, the first mass element having a mass larger than the second mass element, wherein either the first mass element includes one or more permanent magnets and the second mass element includes one or more coils or the second mass element includes one or more permanent magnets and first mass element includes one or more coils, wherein the second mass element is mounted within the first mass element such that the second mass element is free to move within the first mass element and wherein the first mass element is connected to a top of the apparatus via a first spring and/or to a base of the apparatus via a second spring; means for at least partially locking the second mass element to a housing of the apparatus when the first mass element and/or the second mass element has an acceleration below a first threshold, wherein the means for at least partially locking the second mass element to the housing comprises a magnetic locking arrangement; and means for inducing a current in the coils when the second mass element moves relative to the first mass element, wherein: the means for at least partially locking the second mass element to the housing is configured to restrict or prevent transfer of momentum from the first mass element to the second mass element and/or to restrict or prevent relative motion between the first mass element and the second mass element when the first mass element and/or the second mass element has an acceleration below the first threshold, wherein an attractive force of the magnetic locking arrangement is overcome when the acceleration is above the first threshold such that an inertial force on the second mass element overcomes the attractive magnetic force, allowing the second mass element to move freely when impacted by the first mass element; and in use, the first and second mass elements and the housing of the apparatus remain aligned. the means for at least partially locking the second mass element to the housing is configured to restrict or prevent transfer of momentum from the first mass element to the second mass element and/or to restrict or prevent relative motion between the first mass element and the second mass element when the first mass element and/or the second mass element has an acceleration below the first threshold, wherein an attractive force of the magnetic locking arrangement is overcome when the acceleration is above the first threshold such that an inertial force on the second mass element overcomes the attractive magnetic force, allowing the second mass element to move freely when impacted by the first mass element; and in use, the first and second mass elements and the housing of the apparatus remain aligned.