Patent ID: 7378985

Claim:
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; an alternator ( 22 , 63 , 77 , 81 ), an output of which is electrically connected to each of the first and second portions of the drill collar ( 101 ) so as to transmit an electrical signal represented by the output of the alternator into a geological formation being drilled; switch means ( 75 , 79 , 83 , 85 ) for controlling transmission of the electrical signal to the drill collar ( 101 ); and torque generating apparatus ( 2 , 4 , 10 ) which generates torque in response to the electrical output of the alternator and which is mechanically connected to the alternator for transmitting such torque to the alternator for regulating rotation thereof.