Fig. 132 illustrates the wiring diagram of a combination loosely and closely coupled tuner. Two sliding contacts are placed on the primary coil of the receiving transformer and connected with a double pole double throw switch as in the diagram. When the switch is thrown on contacts 1 and 2, the primary is connected to the detector as a double slide tuner, and when on 3 and 4 both the primary and secondary are brought into use as a transformer.
This arrangement may seem cumbersome and is recommended only as a convenience in experimenting. A loosely coupled tuning coil is capable of exact tuning, and unless one understands how to use it, he may not hear a station because the tuner is not properly adjusted. By using the double slide tuner first and then throwing the switch so as to tune in on the transformer, this difficulty may be eliminated.
Fig. 133. Clapp-Eastham Loose Coupler.
Potentiometer.—A potentiometer is not properly classed under the heading of tuning coils, but the construction may be made so similar that it well appears here.
The potentiometer is merely a variable resistance shunted across the terminals of the detector battery in the manner illustrated in the numerous detector circuits. It is used to reduce the voltage of the battery to a value slightly below the critical voltage of the detector. The critical voltage of a detector is the voltage at which its action commences. In the case of an electrolytic detector, it is the voltage required to break down the thin film of gas which collects on the "bare point."
Fig. 134. A Highly Efficient Form of Loose Coupler.
In construction, the potentiometer illustrated in Fig. 135 is in reality a small edition of a double slide tuning coil. It is wound with No. 28 B. S. gauge German silver wire. Three binding posts are mounted on the base, two of them connecting with the ends of the coil and one with the sliding contact.
