\/


the symbols meaning as before.

In words, these formulas mean that, knowing the frequency of the
current and the capacity of a condenser, or the frequency of the
current and the inductance of a circuit (a line or piece of
apparatus), and in either case the resistance of the circuit, one may
learn the impedance by calculation.

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° 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