The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

118 ELECTRO-MAGNETISM.
ceived the idea of covering or insulating the wire, instead of
covering or insulating the iron to be magnetized, as had been
done by others. He effected this by insulating a long wire
with silk thread, and winding this around the rod of iron
in close coils, as is seen in fig. 5, from one end to the other.
The same principle was extended by employing a still longer
insulated wire, and winding several strata of this over the first,
care being taken to insure the insulation between each stratum
by a covering of silk ribbon. By this arrangement the rod
was surrounded by a compound helix, formed of many coils,
instead of a single helix of a fev coils.
In the peculiar arrangement of the coils, Professor Henry
advanced new ideas. Arago and Sturgeon wound their wires
not precisely at right angles to the axis of the rod, as they
should have been, to produce the effect required by the theory
of Ampere, but they were placed obliquely around the rod to be
magnetized; therefore, each turn tended to develop a separate
magnetism not coincident with the axis of the bar. In winding
the wire over itself, as done by Henry, the obliquity of the
several turns compensated each other, and the resultant action
was at right angles to the bar. The ends attained by Henry
were of the greatest importance. The multiplied turns of the
wire, and their peculiar conjunctive action in the generation
of magnetic force in the iron rod, were complete in success.
He found that, after a certain length of wire had been coiled
upon the iron, the power diminished with a further increase of
the number of turns. This was due to the increased resistance
which the longer wire offered to the conduction of electricity.
As an improvement, he increased the number of independent
coils around the u shaped rod, as represented by fig. 6. Another
was to increase the number of cells of the battery to obtain a
current of greater intensity, for the purpose of overcoming the
increased length of the wire, so as to produce or develop the
maximum power of the iron. Fig. 6 represents the manner of
coiling around the iron bar the insulated wire in several independent sections. Each of these sections was united with a
Cruikshank voltaic battery. The experiment proved, that, in
order to produce the greatest amount of magnetism from a
battery of a single cup, a number of helices is required; but
when a compound battery is used, then one long wire must be
employed, making many turns around the iron core. The
magnetic force generated, will be commensurate with the projectile power of the battery.
In describing the results of these experiments, Professor
Henry has used the terms intensity and quantity magnets.