Protective helmet, comprising: - a domed helmet shell; - a support ring (5, 8, 9.1, 9.r) which is fastened to the inside of the helmet shell; - an externally accessible activating element which is rotatably fastened to the helmet shell; and - a transmission unit (10, 11, 12, 15, 16, 25); wherein the support ring (5, 8, 9.1, 9.r) - completely or at least partially surrounds the head of a wearer of the protective helmet; and - establishes a head width of the protective helmet; wherein the transmission unit (10, 11, 12, 15, 16, 25) transmits a rotation of the activating element to the support ring (5, 8, 9.1, 9.r) in such a manner that the head width provided by the support ring (5, 8, 9.l, 9.r) is varied; wherein the transmission unit (10, 11, 12, 15, 16, 25) comprises - a helmet shell-proximal transmission piece (15.1); and - a support ring-proximal transmission piece (15.2); wherein the two transmission pieces (15.1, 15.2) - conjointly completely or at least partially bridge the spacing between the activating element and the support ring (5, 8, 9.1, 9. r); - extend along a common longitudinal axis and are preferably both rod-shaped; wherein the helmet shell-proximal transmission piece (15.1) - is connected in a rotationally fixed manner to the activating element; and - is linearly movable relative to the activating element in two opposite directions parallel to the common longitudinal axis, characterized in that the support ring-proximal transmission piece (15.2) - is connected in a rotationally fixed manner to the helmet shell-proximal transmission piece (15.1) and mechanically connected to the support ring (5, 8, 9.1, 9.r); and - is linearly movable relative to the helmet shell-proximal transmission piece (15.1) in two opposite directions parallel to the common longitudinal axis; and wherein the spacing between the activating element and the support ring (8, 9.l, 9.r) is variable - by a linear movement of the helmet shell-proximal transmission piece (15.1) relative to the activating element as well as - by a linear movement of the support ring-proximal transmission piece (15.2) relative to the helmet shell-proximal transmission piece. - a domed helmet shell; - a support ring (5, 8, 9.1, 9.r) which is fastened to the inside of the helmet shell; - an externally accessible activating element which is rotatably fastened to the helmet shell; and - a transmission unit (10, 11, 12, 15, 16, 25); wherein the support ring (5, 8, 9.1, 9.r) - completely or at least partially surrounds the head of a wearer of the protective helmet; and - establishes a head width of the protective helmet; wherein the transmission unit (10, 11, 12, 15, 16, 25) transmits a rotation of the activating element to the support ring (5, 8, 9.1, 9.r) in such a manner that the head width provided by the support ring (5, 8, 9.l, 9.r) is varied; wherein the transmission unit (10, 11, 12, 15, 16, 25) comprises wherein the transmission unit (10, 11, 12, 15, 16, 25) transmits a rotation of the activating element to the support ring (5, 8, 9.1, 9.r) in such a manner that the head width provided by the support ring (5, 8, 9.l, 9.r) is varied; wherein the transmission unit (10, 11, 12, 15, 16, 25) comprises - a helmet shell-proximal transmission piece (15.1); and - a support ring-proximal transmission piece (15.2); wherein the two transmission pieces (15.1, 15.2) - conjointly completely or at least partially bridge the spacing between the activating element and the support ring (5, 8, 9.1, 9. r); - extend along a common longitudinal axis and are preferably both rod-shaped; wherein the helmet shell-proximal transmission piece (15.1) - is connected in a rotationally fixed manner to the activating element; and - is linearly movable relative to the activating element in two opposite directions parallel to the common longitudinal axis, characterized in that the support ring-proximal transmission piece (15.2) - is connected in a rotationally fixed manner to the helmet shell-proximal transmission piece (15.1) and mechanically connected to the support ring (5, 8, 9.1, 9.r); and - is linearly movable relative to the helmet shell-proximal transmission piece (15.1) in two opposite directions parallel to the common longitudinal axis; and wherein the spacing between the activating element and the support ring (8, 9.l, 9.r) is variable - by a linear movement of the helmet shell-proximal transmission piece (15.1) relative to the activating element as well as - by a linear movement of the support ring-proximal transmission piece (15.2) relative to the helmet shell-proximal transmission piece.