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OUR PHYSICAL WORLD 



circuit shows the type commonly employed on simple receiving 

 circuits, and is in reality two circuits. This method of connection 

 does not introduce the resistance of the head set into the antenna 

 circuit, and permits the vibrations from the antenna to flow more 

 freely. 



The tuning coil (/) consists of one layer of insulated wire (about 

 No. 20) on a cardboard tube about 5 or 6 inches long (Fig. 119). 

 The insulation is removed in two strips on opposite sides of the 

 coil to permit connection by a slider which touches one turn of 

 wire at a time. In this manner the number of turns of wire 

 between the antenna and the ground can be varied by moving 



FIG. 119. A two-slide tuning coil 



the slider. The turns of wire themselves constitute the con- 

 denser in this case so that the capacity is also varied when the 

 slider is moved. This circuit from the antenna (a) (Fig. 118) 

 through the turns of the tuning coil (/) to the ground (g) consti- 

 tutes the primary circuit. 



The secondary circuit uses the same coil but a different portion 

 of it, part of which may overlap the primary inductance, as 

 shown in the diagram. This closed circuit is from the second 

 slider on the tuning coil (/) through the detector (d), through 

 the phones (r), and back to the other end of the tuning coil. 



By moving these sliders we can change the inductance in 

 both the primary and secondary circuits, and thus place the 

 receiving outfit in tune or in electrical resonance with the send- 



