The application describes a rotatable anode for an X-ray tube, wherein the anode comprises a first unit (901) adapted for being hit by a first electron beam at least a second unit (902) adapted for being hit by at least a second electron beam, wherein the first unit and the at least second unit are electrically isolated from each other. Further, the application describes an X-ray system, wherein the system comprises an anode according to the specification, a main cathode for generating an electron beam, wherein the main cathode is adapted to generate a first electrical potential, an auxiliary cathode for influencing a second electrical potential, wherein the main cathode is adapted to deflect the electron beam in order to heat the auxiliary cathode. Furthermore, the application shows a device for determining an electrical potential by detecting the point of impact of an electron beam onto an anode according to the specification and/or by detecting an X-ray spectrum of radiation starting from an anode according to the specification, wherein the electron beam is generated by a cathode, wherein the electron beam hits the first unit of the anode at the point of impact, wherein the electron beam can be deflected, wherein the deflected electron beam hits the second unit of the anode at the point of impact, wherein the first unit and / or second unit emit the radiation.