A probe or measuring head for measuring an electrical signal of an electrical contact has an electrically conducting feeler (1) which protrudes out of a housing (3) and has, at its end, a contact region (4) which comes into touching contact with the electrical contact. The contact region (4) of the feeler (1) and the measurement environment surrounding the contact region (4) of the feeler (1) is illuminated by the light beams from a light source (101, 102, . . . , 10n, 10o) connected to the probe or measuring head.