Electrodes which are cut out of sheet metal possess the disadvantage that they are not quite as efficient and will not last as long as electrodes which are cast.
Cast electrodes are much heavier and far more efficient in many other ways. They cannot be easily made by the young experimenter, but may be procured from any one of several firms dealing in supplies for experimenters.
The right hand sketch in Figure 52 shows how the electrodes should appear when they are mounted in position on the underside of the cover. They are placed about two and one-half inches to three inches apart.
The solution is formed by dissolving sodium-phosphate in water until a "saturated solution" is formed, that is, until the water will not dissolve any more. Sodium-phosphate dissolves rather slowly and it will be necessary to stir the solution and crush the lumps which form with a stick or glass rod.
Fill the jars nearly to the top and then place the electrodes into position.
[Illustration: FIG 51.—A Cast Electrode will last much longer than one cut from Sheet Metal. Cast Electrodes like that above are on the market and can be purchased very reasonably.]
The action of the electrolytic rectifier, in changing alternating current into direct current, is interesting and peculiar. The rectifier acts much like a valve which opens one way and closes the other.
If a battery is connected to the electrodes of a rectifier, the positive pole of the battery being connected to the lead or iron electrode and the negative of the battery to the aluminum electrode of the rectifier, the current from the battery will flow through the rectifier and nothing unusual will happen. If, however, the poles are reversed so that the positive pole is connected to the aluminum electrode, oxygen gas will form on the aluminum. The action of the oxygen gas is to combine with the aluminum and form a coating of *aluminum oxide* all over the electrode. Aluminum oxide is an insulator and it therefore quickly forms an insulating coating which shields the electrode from the solution and stops the passage of the current. This action is almost instantaneous.
[Illustration: FIG. 52.—A completed single Cell Rectifier. The right hand sketch shows how the Electrodes are mounted on the underside of the cover.]
If the rectifier is connected to an alternating current supply it will act just like a valve permitting the current to flow in one direction but stopping it whenever the aluminum electrode is *positive*. The resulting current is therefore, under proper conditions, an intermittent current flowing only in one direction.