HENRY
HENRY
few coils of insulated wire. At a great expense
of battery power it could lift a few pounds; it
was useless iu the arts and inadequate for tele-
graphic purposes. Henry con-
verleil it into two distinct in-
struments. The one, with a
long fine continuous wire,
which is the sensitive instru-
ment, that in the long cir-
cuit of the telegraph responds
to distant influence, Henry
'© ^ _ *• called an intensity' magnet,
becauso to act tlius at a dis-
tance it must be connected with an intensity
battery. The other, wound with many separate
coils of short thick wire, was incapable of action
at a distance, but could be endowed with great
strength. In 1831 he constructed a magnet
capable of sustaining a weight of 3G00 pounds
when excited by the current from a single cell
battery occupying less than one cubic foot of
space. Tiiis he called a quantity magnet because
it required a (quantity battery, and it is this
magnet, with its especial battery, which forms
the short local circuit of the telegraph. The
necessary connection of the magnets with their
respective batteries forms the independent dis-
coverv of Henry of the law of proportion between
the projectile force of the battery and the resist-
ance in the wire of the magnet and length of
circuit. Henry published an account of liis mag-
nets and pointed out the practical application of
his principles to the telegraph in 1831 in the
American Journal of Science. The same year he
transmitted signals through a wire over a mile
long, causing a bell to ring at the further end of
the wire. This length of wire was sufficient to
illastrate to his pupils his principles which in
1898 allowed a message to be sent around the
world. At Princeton, Henry stretched his tele-
graphic wires across the college gi'ounds, com-
municating with Mrs. Henry in his residence.
He made there, in 1833, his largest magnet,
called "Big Ben " by the students, and showed
how an intensity magnet, when excited by dis-
tant influence, might be made to open or close
the circuit of this powerful quantity magnet,
bringing it thus into action at a distance. This
device of opening one circuit by means of another
is used in the telegraph in the relay, to call into
action, to continue the line to another intensity
circuit, and to bring into play the local quantity
circuit. It was used by Henry from 1833 at
Princeton, having been perhaps invented by him
earlier. He showed by it how powerful effects
might be produced at a distance, causing his
magnet in the laboratory to lift and drop thou-
sands of pounds by electrical communication
from his residence on the opposite side of the
college grounds, the same principle as was sub-
sequently used in working machinery from a
distance. In his telegraphic experiments Henry
used the earth for the return current at least as
early as 1835, being the first to complete the
circuit in this way. In 1837 Wheatstorie and
Cook were struggling vainly with tiie telegraph,
having discarded the electro-magnet, an essential
element of their project, finding it incapable of
action at a distance. Henry, then in London,
came to their assistance w-ith liis two magnets
and explained to Wheatstone the principles of
his combinations, in this waj' rendering practi-
cable the telegraph iu England. On his return
to America he aided Morse through personal
interviews, by the exhibition of his own tele-
graphic ai>paratus in Princeton, and by advice
and sympathy by letter, until the telegraph was
introduced. The Morse instrument was a local
tj'pe recorder; it could not act at a distance until
Dr. Leonard T. Gale applied Henry's principles
and made the invention practicable. Not essen-
tial to the telegraph, it was superseded by the
phonetic system of hitting a metal sounder by
means of a moving bar. the simple device of
Henry in his Albany telegraph of 1831, in which
a bar of iron, vibrating between the poles of an
electro-magnet, strvick a bell. All the essential
elements of the electro-magnetic telegraph, viz.,
the phonetic device of liitting a metal sounder,
the two magnets, the one in the long, the other
in the shoi-t circuit, their connection with their
respective batteries, the opening of one circuit
by means of another, — could have been patented
by Henry as early as 1833 had he been so inclined,
but he refused to tie up for his own use discov-
eries which he hoped might benefit the world.
In 1831 he invented his electro-magnetic engine
for maintaining continuous motion by means of
an automatic pole-changer, which proved an im-
portant step in the development of the art of
converting the electric current into mechanical
power. Henry entered the field of the induction
of currents as early as 1827, obtainingsparks from
a common magnet before he made his electro-
magnets, thus making the discovery of magneto-
electricity before Faradaj-, who announced it in
1832. In 1829 or 1830 he discovered the "extra
current," discovered by Faraday in 1834. In 1831
he obtained induced currents and sparks with
his electro-magnets, as he had with the common
magnets, and winding a reel with a mile of wire
to be revolved between the arms of a huge mag-
net, anticipated the dynamo. In 1832 he made
other valual)le experiments in this line and
published his first paper upon the subject in
the Proceedinqs of tiie American Piiilosophical
society. In 1834 he pursued in Princeton the
subject of the " extra current " with copper rib-
