CHAPTER IV. HOW AN ALTERNATING CURRENT MAY BE CHANGED INTO DIRECT CURRENT BY MEANS OF AN ELECTROLYTIC RECTIFIER.
Oftentimes the only source of electrical energy for experimental work is the 110 volt alternating current supply. This may be reduced to a voltage suitable for operating small battery motors, trains, lamps, etc., by means of a "lamp bank" resistance or a step-down transformer.
Direct current is *necessary* however in order to recharge storage batteries and to operate many other devices. The electrolytic rectifier is a device for changing alternating current into direct current and will be found satisfactory for this purpose, provided too much is not demanded of it. It is fairly efficient if used only to rectify small amounts of current. It is not efficient when large amounts of currents are passed through it and quickly becomes very hot in such a case.
An electrolytic rectifier consists of an electrode of iron or lead and one of aluminum immersed in a solution of sodium phosphate.
[Illustration: FIG. 49.—A Single Cell of Electrolytic Rectifier.]
An ordinary glass battery jar may be used to hold the solution, preferably one measuring 5 x 7 inches. The electrodes are supported by a wooden cover which also serves to prevent the solution from evaporating. The cover may be circular in form and simply rest on the top of the jar or may have a groove turned on the underside so that it fits the rim of the jar snugly.
It is a wise precaution to thoroughly saturate the cover with paraffin by immersing it in a molten bath of that material. If the cover is allowed to remain in the molten paraffin until all bubbles have ceased to rise, the paraffin will thoroughly permeate the wood and protect it from the action of the Chemical solution used in the rectifier.
[Illustration: FIG. 50.—An Electrode cut out of Sheet Metal. The top is bent over at right angles and drilled so that it can be mounted on the underside of the cover.]
As stated above, one electrode may be made of iron or lead. The other should be aluminum. The electrode may be cut out of sheet metal and made in the form of a strip about one and one-half inches wide and six inches long. The top of each electrode is bent over at right angles and bored with a small drill and an 8-32 brass machine screw is passed through the hole and through the cover into the bottom of the binding post mounted on the top of the cover, thereby serving not only to fasten the electrode securely in place to the underside of the cover, but also to establish connection between the electrode and the binding post itself.
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.
An alternating current may be represented by a wavy line drawn above and below a straight line. Every time that the wavy line crosses the straight line it represents an alternation or a reversal of the direction in which the current flows.
[Illustration: FIG. 53.—A Diagram showing how a Rectifier cuts off one-half of the Alternating Current Wave and changes it into Pulsating Direct Current.]
The direct current from an electrolytic rectifier working under proper conditions is practically one-half of the alternating current wave and may be represented by the series of waves marked by "C" in the lower part of the illustration in Figure 53.
It will be noticed that these lines do not cross the straight line and the current therefore does not reverse but flows in one direction only.
When a single cell of rectifier is used on the 110 volt current supply, it should be placed in series with a lamp bank or a step-down transformer so that the current is reduced to lower voltage.
Figure 54 shows how to connect a storage cell in series with a single cell of rectifier and a lamp bank so that the storage cell may be recharged from the alternating current. A step-down transformer will be found more efficient and not quite so wasteful of current as a lamp bank. The rectifier and the storage cell are simply connected in series with the secondary of the step-down transformer in order to secure this result.
[Illustration: FIG. 54.—Circuit showing how a Single Cell of Rectifier should be connected in series with a Lamp Bank to Recharge a Storage Cell. A is the Aluminum Plate and L the Lead or Iron Plate.]
[Illustration: FIG. 55.—Diagram showing the Difference in Current after it has been passed through a Single Cell or Rectifier and after passing through a Four-Cell Rectifier.]
The negative pole of the storage cell must always be connected to the aluminum electrode of the rectifier.
The series of little curved lines at the bottom of Figure 53 which represent the alternating current after it has been changed into direct current by the action of the rectifier, have a space between each two, showing that there are periods during which no current flows but that the current is intermittent and made up of a number of short impulses. One-half of the alternating current is therefore really wasted.
It is possible, by means of four cells of rectifier, to so connect them that "both halves of the alternating current wave are utilized" and the spaces are filled up as shown in the lower part of Figure 55.
[Illustration: FIG. 56.—Diagram showing how a Four-Cell Rectifier is connected. The Alternating Current Source is connected to C and D. The Direct Current is taken off at A and B. The Electrodes marked A, A, A, A are the Aluminum Electrodes. L, L, L, L may be Lead or Iron.]
The current which would normally be in a reverse direction, and therefore below the straight line, has been completely reversed so that it flows in the same direction as that above the line.
Figure 56 shows how to connect four cells of rectifier in order to secure this result.
An electrolytic rectifier composed of cells approximately 5 x 7 inches, as described, will not efficiently handle a current of over two to two and one-half amperes. A four cell rectifier will operate to the best advantage on a voltage over 50, and for that reason it is always best to use a lamp bank or step-down transformer in connection with it.
When the solution in a rectifier becomes hot it will not operate as efficiently as when cold. The solution becomes exhausted after a certain amount of usage and requires renewal. This condition will be indicated by the failure of the rectifier to operate properly. The old solution should then be thrown away and the jars and electrodes washed and cleaned thoroughly before the new solution is poured in.
[Illustration: FIG. 57—A Complete Four-Cell Rectifier connected together and Mounted in a Tray.]