Fig. 122. Rolled Condenser
[View full size illustration.]

There has been developed for use in telephony, however, and its use has since extended into other arts requiring condensers, what is called the rolled condenser. This is formed by rolling together in a flat roll four sheets of thin bond paper, 1, 2, 3, and 4, and two somewhat narrower strips of tinfoil, 5 and 6, Fig. 121. The strips of tinfoil and paper are fed on to the roll in continuous lengths and in such manner that two sheets of paper will lie between the two strips of tinfoil in all cases. Thin sheet metal terminals 7 and 8 are rolled into the condenser as it is being wound, and as these project beyond the edges of the paper they form convenient terminals for the condenser after it is finished. After it is rolled, the roll is boiled in hot paraffin so as to thoroughly impregnate it and expel all moisture. It is then squeezed in a press and allowed to cool while under pressure. In this way the surplus paraffin is expelled and the plates are brought very close together. It then appears as in Fig. 122. The condenser is now sealed in a metallic case, usually rectangular in form, and presents the appearance shown in Fig. 123.

Fig. 123. Rolled Condenser
[View full size illustration.]

A later method of condenser making which has not yet been thoroughly proven in practice, but which bids fair to produce good results, varies from the method just described in that a paper is used which in itself is coated with a very thin conducting material. This conducting material is of metallic nature and in reality forms a part of the paper. To form a condenser of this the sheets are merely rolled together and then boiled in paraffin and compressed as before.

Sizes. The condensers ordinarily used in telephone practice range in capacity from about 1/4 microfarad to 2 microfarads. When larger capacities than 2 microfarads are desired, they may be obtained by connecting several of the smaller size condensers in multiple. Table IX gives the capacity, shape, and dimensions of a variety of condensers selected from those regularly on the market.