ion following
the preceding thought upon capacity.

[Illustration: Fig. 34. Test of Line with Varying Serial Inductance]

Capacity and inductance depend only on states of matter. Their
reactances depend on states of matter and actions of energy.

In circuits having both resistance and capacity or resistance and
inductance, both properties affect the passage of current. The joint
reaction is expressed in ohms and is called _impedance_. Its value is
the square root of the sum of the squares of the resistance and
reactance, or, Z being impedance,

-------------------------
/ 1
Z = / R^{2} + ----------------
\/ C^{2}[omega]^{2}

and

--------------------------
Z = / R^{2} + L^{2}[omega]^{2}
\/

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 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
el