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.