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.

TABLE IX

Condenser Data

+------------+---------------+-------------------------

--------+
|            |               |      DIMENSIONS IN INCHES       |
|  CAPACITY  |     SHAPE     |----------+----------+-----------+
|            |               |  Height  |   Width  | Thickness |
+------------+---------------+----------+----------+--------

---+
|    2 m. f. |  Rectangular  |  9-1/6   |  4-3/4   |   11/16   |
|    1 m. f. |       "       |  9-1/6   |  4-3/4   |   11/16   |
|    1 m. f. |       "       |  4-3/4   |  2-3/32  |   13/16   |
|  1/2 m. f. |       "       |  2-3/4   |  1-1/4   |    3/4    |
|    1 m. f. |       "       |  4-13/16 |  2-1/32  |   25/32   |
|  1/2 m. f. |       "       |  4-3/4   |  2-3/32  |   13/16   |
| 3/10 m. f. |       "       |  4-3/4   |  2-3/32  |   13/16   |
|    1 m. f. |       "       |  2-3/4   |  3       |     l     |
+------------+---------------+----------+----------+--------

---+

Conventional Symbols.  The conventional symbols usually employed to represent condensers in telephone diagrams are shown in Fig. 124.  These all convey the idea of the adjacent conducting plates separated by insulating material.

[Illustration:  Fig. 124.  Condenser Symbols]

Functions.  Obviously, when placed in a circuit a condenser offers a complete barrier to the flow of direct current, since no conducting path exists between its terminals, the dielectric offering a very high insulation resistance.  If, however, the condenser is connected across the terminals of a source of alternating current, this current flows first in one direction and then in the other, the electromotive force in the circuit increasing from zero to a maximum in one direction, and then decreasing back to zero and to a maximum in the other direction, and so on.  With a condenser connected so as to be subjected to such alternating electromotive forces, as the electromotive force begins to rise the electromotive force at the condenser terminals will also rise and a current will, therefore, flow into the condenser.  When the electromotive force reaches its maximum, the condenser will have received its full charge for that potential, and the current flow into it will cease.  When the electromotive force begins to fall, the condenser can no longer retain its charge and a current will, therefore, flow out of it.  Apparently, therefore, there is a flow of current through the condenser the same as if it were a conductor.