. Deep borehole surveys and problems. Boring. 112 DEEP BOREHOLE SURVEYS AND PROBLEMS. rent variation in resistance wires which dipped into a bath of mercury, the immersion of^ the wires deciding the resistance. The direction of the deviation was obtained at the same time by a freed needle. Two steel cylinders a and b (Fig. 61) are assembled, b inside a, with a fitting closely in the borehole and with b fixed to the boring rods. The lower part is filled with mercury which, when the apparatus is in the perpendicular position, reaches up to the lower edge e of the upper- most of the two insulated
. Deep borehole surveys and problems. Boring. 112 DEEP BOREHOLE SURVEYS AND PROBLEMS. rent variation in resistance wires which dipped into a bath of mercury, the immersion of^ the wires deciding the resistance. The direction of the deviation was obtained at the same time by a freed needle. Two steel cylinders a and b (Fig. 61) are assembled, b inside a, with a fitting closely in the borehole and with b fixed to the boring rods. The lower part is filled with mercury which, when the apparatus is in the perpendicular position, reaches up to the lower edge e of the upper- most of the two insulated strips e and /. Another insulated strip is at g. The iron wires h and hi are insulated down to a short distance (about 1 in.) over the mercury and joined together in a copper wire i which goes through the rods to bank where it is connected to a tangent galvanometer and battery. If we now turn the rods slowly in a hole deviating from the perpendicular, the thin piece of wire k, between e and /, emerges partially from the mercury. In this way the resistance to the passage of current will be increased and accordingly the deflection of the galvanometer will be diminished. When this deflection has reached a mini- mum, e and g lie in the plane of greatest inclination of the borehole. From the difference between the maximum and minimum throws of the galvanometer we may determine, once for all, what resistance a given length of wire k offers and so get the inclined position of the hole. A rubber boat I floats on the mercury carrying a magnetic needle. This float is guided by rods mm. The hard rubber ring R stand- ing up on two points at about 90 deg. to the piece of wire k is horizontal if e, f and g lie in the plane of greatest inclina- tion of the hole. A steel wire encircles the lower side of R and is intersected by e and / at opposite places and is ^See also Redmayne, R. A. S., "Modern Practice in Mining," Vol. 1, p. 177, and F. Freise, "Stratameter," p. 43, 19
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