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 first method is clearly apparent.
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.
Fig. 102. Increasing the Sensitiveness of an Electrolytic Detector.
Branly discovered that a fine stream of gas passed through the electrolyte in order to agitate it increases the strength of the signals in the phones. He devised a detector provided with two extra platinum terminals sealed in the cup. When connected in series with a battery and an adjustable resistance, these terminals cause electrolysis of the water, and a fine stream of oxygen and hydrogen gas flows through the acid electrolyte. The stream of gas agitates the liquid just sufficiently so that when oscillations strike the detector they augment the breaking down of the film of gas which collects on the fine platinum point. This results in an increase in the battery current flowing through the telephone receivers of from two to four times and a corresponding increase in the volume of sound. The adjustable resistance is used to regulate the decomposition of the electrolyte and formation of gas, for if this proceeds too rapidly an undesirable rumbling noise will be produced in the telephone receivers.
TANTALUM DETECTOR.
The tantalum detector is especially suitable for the amateur experimenter because its change in resistance when struck by oscillations is so great that high resistance telephone receivers are not necessary. Its normal resistance is about 1000-2000 ohms, and this sometimes drops as low as 125-100 ohms when struck by oscillations of ordinary strength. It is not nearly so sensitive as the electrolytic or crystal detectors, but gives very much louder tones in the telephone receivers when used for short distance work.