Obtain two copper rods and lay them across the tank. Cut two pieces of sheet copper having a tongue at each of two corners so that they can be hung in the solution, as shown in Figure 313. Hang both of the sheets from one of the copper rods. Connect this rod to the _positive_ pole of the battery. These sheets are known as the anodes. Then if a piece of carbon, or some metallic object is hung from the other rod and connected to the _negative_ pole of the battery, the electro-plating will commence. The apparatus should be allowed to run for about half an hour and then the object hung from the rod connected to the negative pole of the battery should be lifted out and examined. It will be found thickly coated with copper. It is absolutely necessary to have the poles of the battery connected in the manner stated, or no deposit of copper will take place. Objects which are to be electro-plated must be free from all traces of oil or grease and absolutely clean in every respect, or the plating will not be uniform, because it will not stick to dirty spots. [Illustration: Fig. 313.—A Glass Jar arranged to serve as an Electro-Plating Tank.] Such articles as keys, key-rings, tools, etc., can be prevented from rusting by coating with nickel. Nickel-plating is very similar to copper-plating. Instead, however, of having two copper sheets suspended from the rod connected to the positive pole of the battery, they must be made of nickel. The electrolyte is composed of one part of nickel-sulphate dissolved in twenty parts of water to which one part of sodium-bisulphate is added. This mixture is placed in the tank instead of the copper-sulphate. The objects to be plated are hung from the copper rod connected to the negative pole of the battery. When the nickel-plated articles are removed from the bath they will have a dull, white color known as "white nickel." When white nickel is polished with a cloth wheel revolving at high speed, and known as a buffing-wheel, it will assume a high luster. HOW TO MAKE A RHEOSTAT It is often desirable to regulate the amount of current passing through a small lamp, motor, or other electrical device operated by a battery. This is accomplished by inserting resistance into the circuit. A rheostat is an arrangement for quickly altering the amount of resistance at will. A simple rheostat is easily made by fitting a five-point switch such as that shown in Figure 95 with several coils of German-silver resistance wire. German silver has much more resistance than copper wire, and is used, therefore, because less will be required, and it will occupy a smaller space. A five-point switch will serve satisfactorily in making a rheostat, but if a finer graduation of the resistance is desired it will be necessary to use one having more points.