| 摘要 |
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 sign al to the drill collar (101) such that the electrical signal is transmitted int o a geological formation being drilled, characterised in that the means for generating the electrical signal comprises an alternator (22, 63, 77, 81) an d means (2, 4, 10) responsive to an electrical output of the alternator for regulating rotation of the alternator.
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