Fig. 90. Parts of Universal Detector.
The brass bed plate, B, is 1/8 inch thick, 3 inches long and 2 inches wide. Two holes are drilled on the center line 9/16 inch and 7/8 inch from either end. One is 3/8 inch in diameter and the other is threaded with an 8-32 tap. An insulating bushing in the shape of a hard rubber washer on the lower end of R, 1/8 inch thick and 3/8 inch in diameter, has a 3/16-inch hole bored in its center and is fitted in the larger hole in the bedplate. The whole detector is assembled and mounted on a hard rubber base 4 x 5 x 4 inches. A brass binding rod, M, 3 1/4 inches long and having an 8-32 thread, is passed successively through the arm, the brass standard, the spring, the hard rubber standard and the bedplate. A hexagonal brass nut on the under side of the base and a thumb nut on the brass arm serves to bind the whole tightly together. Four binding posts are mounted on the four corners of the base. Two are connected to the brass binding rod and two to the bedplate. This completes the universal part of the detector. The remaining parts are each described under the headings of the respective detectors to which they belong.
ELECTROLYTIC DETECTORS.
"Bare Point" Type.—Although the electrolytic is the oldest of a long line of very sensitive detectors,[3] it still holds first rank when in the hands of an experienced and skillful operator. It exists in two different forms, but the more favored is that known as the Fessenden "bare point" type, which consists of a very fine Woolaston platinum wire (.001-.00002 of an inch in diameter) dipping in a small cup of dilute acid. The acid is either 20 per cent chemically pure nitric or sulphuric.
Fig. 91. Bare Point Electrolytic Detector.
A large electrode of platinum wire or foil dips into or is sealed in the bottom of the cup so as to make an electrical connection with the liquid. The fine Woolaston wire is clamped over the cup in a holder which permits of vertical adjustment, by means of a thumbscrew, so that the depth of immersion in the acid may be regulated.
Fig. 92. Effect of Exposing too much Wire.
Woolaston wire is covered with a comparatively thick coating of silver, which before using must be removed from the end for about 1/32 inch by dipping it in strong nitric acid, which will dissolve the silver and expose the almost invisible platinum core. Too much of the fine platinum core must not be exposed or else the surface tension of the acid will cause the wire to curl over and present a large flat surface instead of a fine point. This is a very necessary and important precaution, for the detector is more sensitive as the area of contact between the fine wire and the liquid is smaller.