What you have is a variation on the "Loose Coupler" type of crystal set. It's also called a "variometer" because the coupling between the primary and secondary coils can be adjusted mechanically.http://www.sparkmuseum.com/COUPLERS.HTM
One coil is connected to the detector circuit, the other to the antenna. The variable capacitor may be connected on either side, depending on the design.
Crystal radios don't have "gain" as there is no power supply or amplifying element. A long antenna and a ground will undoubtedly improve reception.
What is the detector... a galena crystal with catwhisker or a modern diode?
For more info on crystal sets, try Dave Schmarder's "Make a Radio" site.