A piece of brass tubing, _P_, 1 1/4 inches long and having an internal bore of 1/2 inch is slipped over _U_. A slot cut in _P_ fits over a small pin set in _U_ and permits _P_ to be slid up and down but not to turn around.
[Illustration: Fig. 100. Electrolytic Detector.]
A head in the form of a circular brass washer, _E_, 1/8 inch thick, 1/2 inch in diameter and having a 5/32-inch hole bored in the center is soldered in the top of the tube, _P_.
A circular piece of hard rubber, _H_, 2 inches in diameter and 1/2 inch thick is fitted with a brass bushing having a hole in the center with an 8-32 thread to screw on the rod, _R_.
A spiral spring is placed around _R_ between _U_ and the head _E_. A small brass washer should be placed between _H_ and _E_ in order to eliminate friction. When _H_ is turned in one direction, the spring will cause _P_ to rise, and when turned in the other direction it will be lowered.
A brass arm, _A_, 1/4 x 1/4 x 1 1/2 inches carries a small thumbscrew, _r_, at one end, while the other end is soldered to _P_ as shown in Fig. 96. The Woolaston wire is soldered to _T_.
[Illustration: Fig. 101. Details of Electrolytic Detector.]
A small carbon cup 3/4 x 3/4 inch serves to hold the electrolyte. A 3/8-inch hole is bored 1/4 inch deep in the bottom of the cup and poured full of melted lead. The lead is then bored and tapped to fit a machine screw which fastens the cup to the base. Connection is made from a binding post to the machine screw. A second binding post is connected to the screw which fits into the bottom of _U_ and holds it to the base.
If desirable a circular piece of hard wood, _F_, may be turned out and glued to the base around the cup in order to give it a more finished appearance.
The thumbscrew, _T_, is used to lower the "bare point" until it almost touches the liquid, and then the large head, _H_, is brought into play to make the finer adjustment.
*Increasing the Sensitiveness of an Electrolytic Detector.* The sensitiveness of an electrolytic detector may be increased in three ways, viz., by connecting two detectors in series, by warming the electrolyte and by agitating it.
The second is accomplished by placing the detector over a sand bath and gently warming it. It will then show a marked increase in the strength of the signals at a temperature of about 30 C. This increase will continue to rise with the temperature until it reaches a maximum at about 60 C.