Mr. Joule's first investigations were in the field of magnetism. In
1838, at the age of nineteen, he constructed an electro-magnetic engine,
which he described in Sturgeon's "Annals of Electricity" for January
of that year. In the same year, and in the three years following, he
constructed other electro-magnetic machines and electro-magnets of novel
forms; and experimenting with the new apparatus, he obtained results
of great importance in the theory of electro-magnetism. In 1840 he
discovered and determined the value of the limit to the magnetization
communicable to soft iron by the electric current; showing for the case
of an electro-magnet supporting weight, that when the exciting current
is made stronger and stronger, the sustaining power tends to a certain
definite limit, which, according to his estimate, amounts to about
140 lb. per square inch of either of the attracting surfaces.
He investigated the relative values of solid iron cores for the
electro-magnetic machine, as compared with bundles of iron wire; and,
applying the principles which he had discovered, he proceeded to the
construction of electro-magnets of much greater lifting power than any
previously made, while he studied also the methods of modifying the
distribution of the force in the magnetic field.

In commencing these investigations he was met at the very outset, as he
tells us, with "the difficulty, if not impossibility, of understanding
experiments and comparing them with one another, which arises in general
from incomplete descriptions of apparatus, and from the arbitrary and
vague numbers which are used to characterize electric currents. Such a
practice," he says, "might be tolerated in the infancy of science; but
in its present state of advancement greater precision and propriety are
imperatively demanded. I have therefore determined," he continues,
"for my own part to abandon my old quantity numbers, and to express my
results on the basis of a unit which shall be at once scientific and