Fig. 99 illustrates a simple form of electrolytic detector which is not so sensitive as that shown in Fig. 100 but is still very serviceable.
The cup is made from the carbon of an old dry cell, the brass connecting cap serving very well to make the connections to. It has a recess about 1/2 inch in diameter and 1/4 inch deep cut in the top to contain the electrolyte. The cup should be about one inch high. A file will smooth up any rough edges and give it a good appearance.
Fig. 99. Simple Electrolytic Detector.
The yoke is made of a piece of 1/8-inch sheet brass about 3/4 inch wide, bent in the shape shown in the illustration. Two small holes are drilled in the feet, to serve to fasten the yoke firmly to the base and also to make connection to.
The adjusting screw may be made from the screw taken from the carbon of an old dry cell. To permit of accurate adjustment, it should be fitted with a large head made from a piece of 1/4-inch hard rubber or fiber cut in a circle about 1 1/4 inches in diameter. Bore a small hole about 1/8 inch in diameter through the center of the head and force it on the screw. A nut screwed on the under side will then clamp it tightly against the brass head. A hole is bored in the center of the yoke and a battery nut which will fit the adjusting screw soldered directly under it.
The platinum wire may be either soldered to the adjusting screw or fastened with tinfoil in the method which has been described.
The cup and yoke are best mounted on a piece of hard rubber 1/2 inch thick, 3 inches wide and 4 inches long. A binding post is placed near each of the four corners.
It is possible to do extremely fine and long distance work with the detector illustrated in Fig. 100. It is so arranged that the "bare point" need not necessarily be revolved when making an adjustment, and so it is possible to place it in a very sensitive condition.
A brass standard, U, 1 1/4 inches long is cut from a piece of 1/2-inch rod. A hole bored in the top and bottom of the standard is threaded with an 8-32 tap. A brass rod, R, 2 inches long is threaded with an 8-32 die throughout its entire length. One end is screwed in the top of U.