Hmm! This a bit tricky Nweke. My understanding is that the medium refers to the chamber in which the contacts are placed. The mechanism which actuates the opening of the contacts could be magnetic, heat, or other. What probably is confusing is the vacuum breaker. My understanding is that the contacts are placed in vacuum, ie. it is vacuum which actuates the closure. Let in air, and the contacts open. So in this case the vacuum breaker is actuated by air (Pneumatic). Kindly correct me if I am wrong.