Considerable care must be used in boring carbon because it is very brittle and easily cracks. Only very light pressure should be used on the drill. The carbon is fastened to a strip of wood, about an inch and one-quarter wide, one-half an inch thick, and a little longer than the top of the glass jar is wide. [Illustration: Fig. 60.—A Method of making a Cell Element from Carbon Rods.] A piece of heavy sheet zinc is fastened on the other side opposite the carbon, with a screw. It is a good idea to paint the screws and the surrounding portions of both the zinc and the carbon with hot paraffin wax so that the solution will not "creep" and attack the screws. It is also a good plan first to soak the wooden strip in some hot paraffin until it is thoroughly impregnated. Ammonium chloride, or, as it is more commonly called, sal ammoniac, should be added to a jar of water until it will dissolve no more. The zinc and carbon elements may then be placed in the solution. One of the great disadvantages of the voltaic cell is that the zinc is attacked by the acid when the battery is not in use and cannot be allowed to remain in the solution without quickly wasting away. This is true in the case of the Leclanche cell only to a very small extent. The voltaic cell is more powerful than the Leclanche cell, but the elements must be carefully lifted out and rinsed with water every time that you are through using the cell. By using several carbon plates instead of one, the cell may be made more powerful. The illustrations show several ways of accomplishing this. The simplest method is to place a carbon plate on each side of the wooden strip and use a zinc in the form of a rod which passes through a hole between the two. Care must always be used to keep any screws which are used to hold the carbons or zincs in position in the cells from touching each other. [Illustration: Fig. 61. An Element made from two Carbon Plates and a Zinc Rod.] In Figure 62 an arrangement of using four carbons is shown. The drawing is self-explanatory. In any of the cells using more than one carbon element, the carbons should all be connected. In discussing the voltaic cell we mentioned the fact that it becomes polarized, and explained this phenomenon as being caused by hydrogen bubbles collecting on the copper or positive pole. The same thing happens in the case of carbon or any other material which is used as a positive. *Polarization* is the "bugbear" of batteries. It can be eliminated to a certain extent, however, by the use of a "depolarizer" _placed in the solution_. There are several such substances, the most common being _sodium bichromate_ and _potassium bichromate_. These are used in battery preparations on the market called "Electric Sand," "Electropoian Fluid," etc. [Illustration: Fig. 62. A Method of Mounting four Carbon Plates.]