you need a second coil as a "receiver" (called the "secondary" of a transformer), and the diodes are connected to the secondary coil to change the high frequency AC into DC. the amount of voltage and current available on the secondary side depends on how much voltage and current are used to drive the primary side. the power (in watts) is calculated as E*I (voltage times current), and will be nearly equal when compared between primary and secondary. i said nearly equal, because there is always a slight amount of loss. if you drive the primary side with 10 volts at 10 amps at 40khz, and the primary and secondary have the same number of turns of wire, and the primary and secondary are really well coupled magenetically, you will be able to get 10 volts at 9 amps out of the secondary (assuming efficiency at 90%... you will never get 100% or more, it will always be less than 100%) to drive the load. you can change the ratio of the turns of wire between the primary and secondary to get more voltage (add secondary turns) or less voltage (subtract secondary turns) and the current available from the secondary will be inversely proportional to the voltage. so you could get 0.9 volts at 100 amperes with a turns ratio of 10:1 or 100 volts at 0.9 amperes with a ratio of 1:10.