Electrostatic Capacity

. It is the po e ion of electrostatic capacity which enables the condenser, of which the Leyden jar is a good example, to be useful in a telephone line. The simplest form of a condenser is illustrated in Fig. 28, in which two conducting surfaces are separated by an insulating material. The larger the surfaces, the closer they are together; and the higher the specific inductive capacity of the insulator, the greater the capacity of the device. An insulator used in this relation to two conducting surfaces is called the dielectric . IMG:384091999325875926_fig028_t.gif.png:Illustration_ Fig. 28. Simple Condenser Fig. 28. Simple Condenser View full size illustration. IMG:384091999325875926_fig029_t.gif.png:Illustration_ Fig. 29. Condenser Symbols Fig. 29. Condenser Symbols View full size illustration. Two conventional signs are used to illustrate condensers, the upper one of Fig. 29 growing out of the original condenser of two metal plates, the lower one suggesting the thought of interleaved conductors of tin foil, as for many years was the practice in condenser construction. With relation to this property, a telephone line is just as truly a condenser as is any other arrangement of conductors and insulators. A ume such a line to be open at the distant end and its wires to be well insulated from each other and the earth. Telegraphy through such a line by ordinary means would be impo ible. All that the battery or other source could do would be to cause current to flow into the line for an infinitesimal time, raising the wires to its potential, after which no current would flow. But, by virtue of electrostatic capacity, the condition is much as shown in Fig. 30. The condensers which that figure shows bridged acro the line from wire to wire are intended merely to fix in the mind that there is a path for the transfer of electrical energy from wire to wire. IMG:384091999325875926_fig030_t.gif.png:Illustration_ Fig. 30. Line with Shunt Capacity Fig. 30. Line with Shunt Capacity View full size illustration. A simple test will enable two of the results of a short-circuiting capacity to be appreciated. Conceive a very short line of two wires to connect two local battery telephones. Such a line po e es negligible resistance, inductance, and shunt capacity. Its insulation is practically infinite. Let condensers be bridged acro the line, one by one, while conversation goes on. The listening observer will notice that the sounds reaching his ear steadily grow le loud as the capacity acro the line increases. The speaking observer will notice that the sounds he hears through the receiver in series with the line steadily grow louder as the capacity acro the line increases. Fig. 31 illustrates the test. The speaker's observation in this test shows that increasing the capacity acro the line increased the amount of current entering it. The hearer's observation in this test shows that increasing the capacity acro the line decreased the amount of energy turned into sound at his receiver. IMG:384091999325875926_fig031_t.gif.png:Illustration_ Fig. 31. Test of Line with Varying Shunt Capacity Fig. 31. Test of Line with Varying Shunt Capacity View full size illustration. The unit of electrostatic capacity is the farad . As this unit is inconveniently large, for practical applications the unit microfarad —millionth of a farad—is employed. If quantities are known in microfarads and are to be used in calculations in which the values of the capacity require to be farads, care should be taken to introduce the proper corrective factor. The electrostatic capacity between the conductors of a telephone line depends upon their surface area, their length, their position, and the nature of the materials separating them from each other and from other things. For instance, in an open wire line of two wires, the electrostatic capacity depends upon the diameter of the wires, upon the length of the line, upon their distance apart, upon their distance above the earth, and upon the specific inductive capacity of the air. Air being so common an insulating medium, it is taken as a convenient material whose specific inductive capacity may be used as a basis of reference. Therefore, the specific inductive capacity of air is taken as unity. All solid matter has higher specific inductive capacity than air. The electrostatic capacity of two open wires.165 inch diameter, 1 ft. apart, and 30 ft. above the earth, is of the order of.009 microfarads per mile. This quantity would be higher if the wires were closer together; or nearer the earth; or if they were surrounded by a gas other than the air or hydrogen; or if the wires were insulated not by a gas but by any solid covering. As another example, a line composed of two wires of a diameter of.036 inch, if wrapped with paper and twisted into a pair as a part of a telephone-cable, has a mutual electrostatic capacity of approximately.08 microfarads per mile, this quantity being greater if the cable be more tightly compre ed. The use of paper as an insulator for wires in telephone cables is due to its low specific inductive capacity. This is because the insulation of the wires is so largely dry air. Rubber and similar insulating materials give capacities as great as twice that of dry paper. The condenser or other capacity acts as an effective barrier to the steady flow of direct currents. Applying a fixed potential causes a mere rush of current to charge its surface to a definite degree, dependent upon the particular conditions. The condenser does not act as such a barrier to alternating currents, for it is po ible to talk through a condenser by means of the alternating voice currents of telephony, or to pa through it alternating currents of much lower frequency. A condenser is used in series with a polarized ringer for the purpose of letting through alternating current for ringing the bell, and of preventing the flow of direct current. The degree to which the condenser allows alternating currents to pa while stopping direct currents, depends on the capacity of the condenser and on the frequencies of alternating current. The larger the condenser capacity or the higher the frequency of the alternations, the greater will be the current pa ing through the circuit. The degree to which the current is opposed by the capacity is the reactance of that capacity for that frequency. The formula is Capacity reactance = 1 ÷ C  ω wherein C is the capacity in farads and ω is 2 π  n , or twice 3.1416 times the frequency. All the foregoing leads to the generalization that the higher the frequency, the le the opposition of a capacity to an alternating current. If the frequency be zero, the reactance is infinite, i.e. , the circuit is open to direct current. If the frequency be infinite, the reactance is zero, i.e. , the circuit is as if the condenser were replaced by a solid conductor of no resistance. Compare this statement with the correlative generalization which follows the next thought upon inductance.