Insulation of Conductors.  The fourth property of telephone lines, insulation of the conductors, usually is expressed in ohms as an insulation resistance.  In practice, this property needs to be intrinsically high, and usually is measured by millions of ohms resistance from the wire of a line to its mate or to the earth.  It is a convenience to employ a large unit.  A million ohms, therefore, is called a megohm.  In telephone cables, an insulation resistance of 500 megohms per mile at 60 deg.  Fahrenheit is the usual specification.  So high an insulation resistance in a paper-insulated conductor is only attained by applying the lead sheath to the cable when its core is made practically anhydrous and kept so during the splicing and terminating of the cable.

Insulation resistance varies inversely as the length of the conductor.  If a piece of cable 528 feet long has an insulation resistance of 6,750 megohms, a mile (ten times as much) of such cable, will have an insulation resistance of 675 megohms, or one-tenth as great.

Inductance vs.  Capacity.  The mutual capacity of a telephone line is greater as its wires are closer together.  The self-induction of a telephone line is smaller as its wires are closer together.  The electromotive force induced by the capacity of a line leads the impressed electromotive force by 90 degrees.  The inductive electromotive force lags 90 degrees behind the impressed electromotive force.  And so, in general, the natures of these two properties are opposite.  In a cable, the wires are so close together that their induction is negligible, while their capacity is so great as to limit commercial transmission through a cable having .06 microfarads per mile capacity and 94 ohms loop resistance per mile, to a distance of about 30 miles.  In the case of open wires spaced 12 inches apart, the limit of commercial transmission is greater, not only because the wires are larger, but because the capacity is lower and the inductance higher.

Table I shows-the practical limiting conversation distance over uniform lines with present standard telephone apparatus.

TABLE I

Limiting Transmission Distances

+-----------------------------+----------------------+
|   SIZE AND GAUGE OF WIRE    |  LIMITING  DISTANCE  |
+-----------------------------+----------------------+
| No.  8 B. W. G. copper      |      900 miles       |
|     10 B. W. G. copper      |      700 miles       |
|     10 B. &  S. copper      |      400 miles       |
|     12 N. B. S. copper      |      400 miles       |
|     12 B. &  S. copper      |      240 miles       |
|     14 N. B. S. copper      |      240 miles       |
|      8 B. W. G. iron        |      135 miles       |
|     10 B. W. G. iron        |      120 miles       |
|     12 B. W. G. iron        |       90 miles       |
|  16 B. &  S. cable, copper  |       40 miles       |
|  19 B. &  S. cable, copper  |       30 miles       |