Circuit breaker (1) insulated by a dielectric fluid which comprises a first pair of contacts (10, 20), which can move in translation with respect to one another, an arc extinction chamber (30), a compression chamber (40) suitable for propelling a part of the dielectric fluid by piston effect into the extinction chamber (30). The circuit breaker (1) is adapted so that the dielectric fluid propelled by the compression chamber (40) is introduced into the extinction chamber (30) through an end of the first contact (10). According to one possibility of the invention, the extinction chamber is surrounded by a fixed thermal chamber perforated with orifices and thus ensuring blasting as close as possible to the arc contacts.