Electrolytic Rectifier Schematic

A kind of electrical one-way valve is formed by a cell with its electrodes made from iron and aluminum or zinc-aluminum alloy and its electrolyte made of a solution of ammonium phosphate.  In actual practice, four of these cells form the rectifier so that both waves of the alternating current may be used.  The connections are shown in the figure at the right. 

Current can flow from the iron to the aluminum without resistance.  Upon a reversal of the direction, when current attempts to pass from the aluminum to the iron, the aluminum electrode immediately becomes covered with aluminum hydroxide film. This compound is an electrical insulator that prevents the passage of electricity in this direction.  As soon as the current again flows from the iron to the aluminum, the hydroxide film is broken down, so the resistance vanishes.

Considering the connections diagram, the alternating current supply is at the top, while the direct current leads are at the bottom.  The direction of the current waves used for the positive side of the direct current is shown in full-line arrows, while the waves used for the negative of the direct current are shown in dotted arrows.  It will be seen that the arrows of the alternating current supply line run in both directions, which is, of course, the actual case with the current impulses.

It will be seen that one side of the alternating circuit is connected with two of the cells in such a manner that it reaches one aluminum and one iron electrode. In contrast, the other side of the alternating supply is similarly connected with the remaining two cells.  Current impulses from an alternating current line will pass to the iron electrode, thence to the aluminum, and, as both aluminum electrodes are connected with the positive side of the direct circuit, the current will always flow from this positive line.

Returning from the negative side of the direct current line, as shown by dotted arrows, the flow passes to the two iron electrodes, thence to the two aluminum electrodes.  One of these aluminum electrodes is connected to one side of the alternating supply, while the other electrode is connected to the other alternating current wire.  The current then returns to the side of the alternating lines opposite the side from which that particular impulse started.  Upon a reversal of the alternating impulse direction, the remaining two cells will be used, but the flow will occur in the same manner as just described.

The above text is copied from Automobile Battery Care and Repair by Harold P. Manly, 1920.