Rolled-Up Condensers.
Now that the condenser has become so important a factor in telephone work, many schemes for cheapening and facilitating their manufacture have been devised. One in particular merits description, the "rolled-up" condenser having come largely into use. The tin-foil is supplied in rolls containing many yards of foil of the requisite width for the condenser to be made. Likewise rolls of paper are provided, exceeding in width, however, those of tin-foil. These rolls are arranged upon horizontal spindles in front of an empty spindle, or mandrel, upon which the condenser is to be formed. A few turns of the paper ribbon are made around the mandrel, then the foil is brought forward and a few turns made, then follows a turn of paper ribbon and another of foil, and finally a paper layer; and the mandrel being rotated, the alternate layers of foil and paper are laid on and rolled around each other on the mandrel until the requisite quantity is obtained. It then becomes an easy matter to cut the paper ends so no contact is possible between the layers of foil. The whole thing is slipped off the mandrel, secured by a rubber band or two, placed in a hot paraffin bath, and left to become saturated while still warm and before the paraffin has time to harden; the cylinder is put under a press and squeezed flat, driving out excess paraffin, and leaving the condenser in a convenient shape to handle. Connections are then made to the foil leaves, and a case of wood or metal completes the work.
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 D3 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.