Fig. 104. United Wireless Carborundum Detector (horizontal type).
In order to use the universal detector for minerals, a special contact similar to that shown in Fig. 90 must be made. The contact is bored and threaded on its under side to fit a brass pin 3/4 inch long and having an 8-32 thread. The other end of the pin screws into the hole in the bedplate. The large knurled portion of the contact permits it to be raised or lowered without the fingers coming in contact with the crystal. The crystal is clamped between the contact and the spring, S. The position is varied until a sensitive spot is found and then the pressure is carefully regulated by means of the large adjusting screw until the signals in the telephone receivers are the loudest. If possible avoid touching the crystals with the fingers, as the oil and dirt, even though it cannot always be seen, spoils their value for long distance work. Use instead a pair of steel forceps.
Fig. 105. United Wireless Carborundum Detector (vertical type).
The United Wireless Telegraph Co. makes use of carborundum in the detectors shown in Figs. 104 and 105. The principal advantage of carborundum over such substances as silicon, etc., is that it is not affected by the heavy discharge of the transmitting apparatus and does not require a new adjustment after each period of sending. All the crystals will not work, and so a large cake should be purchased and the desired crystals selected. The dark blue portions of the mass, which are the hardest, will give the clearest tones in the telephone receivers, and are preferable to the lighter colored crystals. Since the crystals conduct better in one direction than in the other, as explained above, the adjustment must be made with the view of determining in which position the particular crystal will work the best.
Carborundum will produce sounds in the telephone receivers without the aid of any battery, but for careful work a battery and a potentiometer are necessary.
The other crystals given in the column merely require that the telephone receivers be connected to the detector terminals as in the wiring diagram in Fig. 108.
The Clapp-Eastham detector makes use of a crystal of iron pyrites held in a brass retaining cup beneath the metal contact point. It is not affected by strong signals and requires no battery or potentiometer. When adjusted it will remain in a sensitive condition for a long time without further attention.
