To study the diode action.
a low frequency oscillator, a diode, a resistor, sensitive galvanometer
Connect the oscillator to a diode, a resistor and a galvanometer. Look at the needle of the galvanometer. From the middle, it goes to one side and returns to the middle. It does not go on the other side, stays in the middle for some time and again goes in the same direction. This keeps on continuing.
Stop the current, remove the diode, invert it physically and reconnect. Now, you see that the needle deflects in the opposite direction. It goes from the middle to that side and returns.
These observations shows that the diode allows the current to flow only in one direction and not in the opposite direction. Therefore, it acts as a half wave rectifier.