its length, and its temperature.
Conductivity of a copper wire, for example, increases in direct ratio
to its weight, in inverse ratio to its length, and its conductivity
falls as the temperature rises. Resistance is the reciprocal of
conductivity and the properties, conductivity and resistance, are more
often expressed in terms of resistance. The unit of the latter is the
_ohm_; of the former the _mho_. A conductor having a resistance of 100
ohms has a conductivity of .01 mho. The exact correlative terms are
_resistance_ and _conductance_, _resistivity_ and _conductivity_. The
use of the terms as in the foregoing is in accordance with colloquial
practice.

Current in a circuit having resistance only, varies inversely as the
resistance. Electromotive force being a cause, and resistance a state,
current is the result. The formula of this relation, Ohm's law, is

C = E/R

_C_ being the current which results from _E_, the electromotive force,
acting upon _R_, the resistance. The units are: of current, the
ampere; of electromotive force, the volt; of resistance, the ohm.

As the conductivity or resistance of a line is the property of
controlling importance in telegraphy, a similar relation was expected
in early telephony. As the current in the telephone line varies
rapidly, certain other properties of the line assume an importance
they do not have in telegraphy in any such degree.

The importance that these properties assume is, that if they did not
act and the resistance of the conductors alone limited speech,
transmission would be possible direct from Europe to America over a
pair of wires weighing 200 pounds per mile of wire, which is less than
half the weight of the wire of the best long-distance land lines now
in service. The distance from Europe to America is about twice as
great as the present commercial radius by land lines of 435-pound
wire. In other words, good speech is possible through a mere
resistance twenty times greater than the resistance of the longest
actual open-wire line it is possible to talk through. The talking
ratio between a mere resistance and the resistance of a regular
telephone cable is still greater.

Electrostatic Capacity. It is the possession 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