A process for building a tyre, comprising: - providing a resonant circuit of an electromagnetic sensor (30; 30'; 30") wherein the resonant circuit comprises a support of dielectric material resting on a head of an extruder and at least a first conductive element (35; 36) provided on said support in a position spaced apart from said head and capable of generating an electromagnetic field in a region comprising said head, - energizing said resonant circuit, in such a way that said at least a first conductive element (35; 36) generates an electromagnetic field in a region comprising said head of said extruder, - measuring a parameter associated with said resonant circuit and produced in response to said energizing of said resonant circuit, said parameter being a function of the electrical permittivity characteristics of said region, - starting to extrude a continuous elongated element made of elastomeric material through an aperture formed in said head of said extruder, in such a way that said electrical permittivity characteristics of said region are modified, - detecting a variation of said parameter caused by the modification of said electrical permittivity characteristics, thus determining the presence of said continuous elongated element outgoing from said aperture, - putting a forming drum into rotation about its axis of rotation, at the end of a predetermined time interval after said detection of the variation of said parameter, - distributing said continuous elongated element along a radially outer surface of said forming drum, thereby forming at least a portion of an elastomeric material component of a tyre by winding on turns which are in side by side relationship and/or at least partially overlapping. - providing a resonant circuit of an electromagnetic sensor (30; 30'; 30") wherein the resonant circuit comprises a support of dielectric material resting on a head of an extruder and at least a first conductive element (35; 36) provided on said support in a position spaced apart from said head and capable of generating an electromagnetic field in a region comprising said head, - energizing said resonant circuit, in such a way that said at least a first conductive element (35; 36) generates an electromagnetic field in a region comprising said head of said extruder, - measuring a parameter associated with said resonant circuit and produced in response to said energizing of said resonant circuit, said parameter being a function of the electrical permittivity characteristics of said region, - starting to extrude a continuous elongated element made of elastomeric material through an aperture formed in said head of said extruder, in such a way that said electrical permittivity characteristics of said region are modified, - detecting a variation of said parameter caused by the modification of said electrical permittivity characteristics, thus determining the presence of said continuous elongated element outgoing from said aperture, - putting a forming drum into rotation about its axis of rotation, at the end of a predetermined time interval after said detection of the variation of said parameter, - distributing said continuous elongated element along a radially outer surface of said forming drum, thereby forming at least a portion of an elastomeric material component of a tyre by winding on turns which are in side by side relationship and/or at least partially overlapping.