A constant tension device (30; 70; 160; 180), comprising: a carrier configured to be movable along an axis (a); a primary spring attached to the carrier so as to apply a primary spring force (Fp) directed along the axis (a), the primary spring force (Fp) applied to the carrier changing in accordance with a primary spring rate function as the carrier moves relative to the primary spring along the axis (a); a wire or string attached to the carrier and extending along the axis (a) so that a net axial force applied to the carrier is applied to the wire or string ; and a secondary spring having a first end attached to the carrier so as to apply a secondary spring force (Fs) to the carrier , a secondary spring angle (α) being defined between a line (W) normal to the axis (a) and a line of action of the secondary spring , the secondary spring force being directed transverse to the axis (a) and having an axial component (Fsa) that is applied to the carrier in a direction along the axis (a), wherein the secondary spring force (Fs) is configured so that the axial component (Fsa) of the secondary spring force (Fs) varies in accordance with a secondary spring rate function as the carrier moves relative to the primary spring along the axis (a); wherein the net axial force applied to the carrier comprises the sum of the primary spring force (Fp) and the axial component (Fsa) of the secondary spring force (Fs), the constant tension device being characterized in that : the secondary spring is chosen so that as the carrier moves longitudinally along the axis (a) within an operational range the axial component (Fsa) of the secondary spring force (Fs) changes in accordance with the secondary spring rate function, and the secondary spring rate function approximates and opposes the primary spring rate function so that the net axial force maintains the wire or string at a tension at or near a preferred tension (Tp). a carrier configured to be movable along an axis (a); a primary spring attached to the carrier so as to apply a primary spring force (Fp) directed along the axis (a), the primary spring force (Fp) applied to the carrier changing in accordance with a primary spring rate function as the carrier moves relative to the primary spring along the axis (a); a wire or string attached to the carrier and extending along the axis (a) so that a net axial force applied to the carrier is applied to the wire or string ; and a secondary spring having a first end attached to the carrier so as to apply a secondary spring force (Fs) to the carrier , a secondary spring angle (α) being defined between a line (W) normal to the axis (a) and a line of action of the secondary spring , the secondary spring force being directed transverse to the axis (a) and having an axial component (Fsa) that is applied to the carrier in a direction along the axis (a), wherein the secondary spring force (Fs) is configured so that the axial component (Fsa) of the secondary spring force (Fs) varies in accordance with a secondary spring rate function as the carrier moves relative to the primary spring along the axis (a); wherein the net axial force applied to the carrier comprises the sum of the primary spring force (Fp) and the axial component (Fsa) of the secondary spring force (Fs), the constant tension device being characterized in that : the secondary spring is chosen so that as the carrier moves longitudinally along the axis (a) within an operational range the axial component (Fsa) of the secondary spring force (Fs) changes in accordance with the secondary spring rate function, and the secondary spring rate function approximates and opposes the primary spring rate function so that the net axial force maintains the wire or string at a tension at or near a preferred tension (Tp). <EOS>