There is no reason why aluminum foil or lead foil, or, in fact, any thin sheet metal should not be used in condensers. In telephone work, paper covered with gilt paint was tried, and worked fairly well, but was ultimately rejected in favor of tin-foil. In some cases, when it is desired to construct a condenser for high potential work, the oil-tank apparatus can be used. This is readily made of any desired dimensions, as follows: Procure a square glass jar, such as is made for storage batteries, a few pieces of sheet metal cut to fit loosely in the jar, some glass rods and sufficient clean "transformer oil" or heavy paraffin oil to nearly fill the jar. The sheets of metal can then be hung from the glass rods into the jar, being separated one-half inch, and the oil poured in. Two plates, about 8 inches by 6 inches, will hang nicely into a type D^3 Chloride Battery jar, which is 7⅞ inches long by 9½ inches high by 3¼ inches wide. Altering the relative distances between the plates will give considerable adjustment to this simple condenser, or, if desired, more plates may be inserted and connected up, as in the tin-foil condensers. This type can be made portable, but it is not to be recommended unless no objection is had to emptying and refilling the jar with oil.
In operating large coils, it is convenient to be able to vary the capacity of the condenser on the primary circuit. To make an adjustable condenser presents no more difficulty than a non-adjustable one, simply more labor. For example, the large condenser used with the 6-inch spark coil might be divided into four sections, containing 2000 square inches, 500 square inches, 300 square inches, and 200 square inches of surface (see Fig. 34). Wires leading from the ends of the foil sheets _C C_ are to be brought to the brass plates _G G_. The brass rods _B B_ are connected by binding posts to the coil, each strip being well insulated from its neighbor. Any combination is possible by the insertion of brass plugs in holes drilled between the strips. The plugs must be fully large enough to make good contact on each of the two strips between which they are inserted, and should be turned taper. With the largest coils the condenser and contact breaker are generally mounted separately, and are fully adjustable.