A communication system for down hole use and comprising a drill collar (101) comprising a first portion (103) and a second portion (105) separated from each other by an electrically insulating material (67) and means (22, 63, 77, 81) for generating an electrical signal and for applying the electrical signal to the drill collar (101) such that the electrical signal is transmitted into a geological formation being drilled, characterized in that the means for generating the electrical signal comprises an alternator (22, 63, 77, 81) and means (2, 4, 10) responsive to an electrical output of the alternator for regulating rotation of the alternator.