Well logging apparatus

Claim 1

WELL LOGGING APPARTUS, COMPRISING: AN ELECTRICALLY CONDUCTIVE CAISSON HAVING ONE OPEN END; A SEALING PAD AFFIXED TO SAID CAISSON AROUND SAID OPEN END ADAPTED TO ISOLATE THE INTERIOR OF THE CAISSON FROM FLUIDS WITHIN A BOREHOLE WHEN SAID PAD IS PLACED AGAINST THE SIDE OF THE BOREHOLE; AN ELECTRODE HOLDER COMPRISES A BODY MEMBER OF ELECTRICALLY NONCONDUCTIVE MATERIAL HAVING ENLARGED END PORTIONS CONNECTING TOGETHER AN INTERMEDIATE PORTION OF REDUCED DIAMETER; AN OPENING IN SAID CAISSON OPPOSITE SAID OPEN END THROUGH WHICH SAID INTERMEDIATE OPORTION OF SAID ELECTRODE HOLDER EXTENDS; SEALING MEANS BETWEEN SAID CAISSON AND SAID INTERMEDIATE PORTION ADAPTED TO SEAL THE INTERIOR OF THE CAISSON AND PERMIT BILATERIAL MOVEMENT OF SAID ELECTRODE HOLDER THROUGH SAID OPENING; SPRING MEANS POSITIONED BETWEEN SAID CAISSON AND THE ENLARGED END PORTION OF SAID ELECTRODE HOLDER OUTSIDE OF SAID CAISSON ADAPTED TO RETRACT INTO SAID CAISSON THE ENLARGED END PORTION WITHIN SAID CAISSON, AND TO PERMIT MOVEMENT OF SAID ELECTRODE HOLDER OUT OF SAID CAISSON THROUGH SAID OPEN END WHEN THE PRESSURE WITHIN SAID CAISSON IS LESS THAN PRESSURE OF BOREHOLDE FLUIDS ON SAID CAISSON; A METALLIC ELECTRODE ON THE ENLARGED END PORTION OF SAID ELECTRODE HOLDER WITHIN SAID CAISSON AN ELECTRICAL CONNECTOR EXTENDING THROUGH SAID ELECTRODE HOLDER FROM THE ENLARGED END OF SAID HOLDER OUTSIDE OF