For input 12vac left side, connect it to transformer 1000turns coil Qin parallel , for ouptput 6v , 24w light bulb at right side, we need to step down the voltage , use transformer formulae (Vp/Vs) = Np/Ns ,so connect 500 turns coil S to light bulb in parallel , for the center is transfomer coil p and coil R which is 50 , 50 turns , since turns is same in center , no voltage change in center. Voltage change only at right and left side