nd a current of
electricity sent through the wire, the soft iron would become a magnet,
but would retain its magnetism no longer than while the current of
electricity was passing through the coil. The magnetism developed in
this way was called electro-magnetism, and the iron so wound was called
an electro-magnet. The first electro-magnet was made by winding bare
wire upon the soft iron. This method will not produce very strong
magnets. In 1830 Prof. Henry insulated the wire by covering it with
silk, and was the first to produce powerful magnets.

On a soft iron bar of fifty-nine pounds weight he used twenty-six coils
of wire, thirteen on each leg, all joined to a common conductor by their
opposite ends, and having an aggregate length of seven hundred and
twenty-eight feet. This apparatus was found able to sustain a weight of
twenty-five hundred pounds. This electro-magnet is now owned by Yale
College.

The power of the electro-magnet is enormously greater than that of any
permanent magnet. A permanent magnet made by Jamin of Paris, which is
made up of many strips of thin steel bound together, and weighing four
pounds, is able to support a weight of one hundred pounds; but Dr. Joule
made an electro-magnet, by arranging the coils to advantage, that would
support thirty-five hundred times its own weight, or one hundred and
forty times the proportionate load of Sir Isaac Newton's ring magnet.

THE GALVANIC BATTERY.

The original form of the galvanic battery as devised by Volta, and
modified but little during thirty years, consisted of a cell to contain
a fluid, which was usually dilute sulphuric acid, in which two plates of
different metals were immersed: the metals used were generally plates of
zinc and copper, or zinc and silver. Such plates, when first placed in
the liquid, will give a very good current of electricity; but it will
not last long. The reason of this is easy to understand. Whenever a
current of electricity is generated by chemical action of a liquid upon
two different metals, there is always some decomposition of the liquid,
and this decomposition takes place upon the plates themselves; and the
liberated gases _adhere to the plates, and prevent further contact with
the acid_; at the same time, the gases themselves act upon the plates,
and generate a current of electricity in the opposite direction. This
will of course interfere with the first current; and very soon the
battery is useless until the pla