the earth. A lack of skill in preparing the wires for putting in the
earth caused these early underground lines to be failures. At the
urging of one of his associates, Professor Morse consented to place
his earliest telegraph lines on poles in the air. Each such line
originally consisted of two wires, one for the going and one for the
returning current, as was then considered the action. Upon its being
discovered that a single wire, using the earth as a return, would
serve as a satisfactory telegraph line, such practice became
universal. Upon the arrival of the telephone, all lines obviously were
built in the same way, and until force of newer circumstances
compelled it, the present metallic circuit without an earth connection
did not come into general use.

The extraordinary growth of the number of telephone lines in a
community and the development of other methods of electrical
utilization, as well as the growth in the knowledge of telephony
itself, ultimately forced the wires underground again. At the same
time and for the same causes, a telephone line became one of two
wires, so that it becomes again the counterpart of the earliest
telegraph line of Professor Morse.

Another curious and interesting example of this reversion to type
exists in the simple telephone receiver. An early improvement in
telephone receivers after Professor Bell's original invention was to
provide the necessary magnetism of the receiver core by making it of
steel and permanently magnetizing it, whereas Professor Bell's
instrument provided its magnetism by means of direct current flowing
in the line. In later days the telephone receiver has returned almost
to the original form in which Professor Bell produced it and this
change has simplified other elements of telephone-exchange apparatus