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

DISCOVERIES OF STURGEON AND HENRY. 119
By the former is meant, that when a piece of soft iron, so surrounded with wire that its magnetic power could be called
into operation by an intensity battery, the magnet was called
an " intensity magnet;" when it was acted upon by a quantity
battery through a number of separate coils, so that its magnetism could be fully developed, it was.alled a "quantity
magnet." The terms are technical, and very appropriate.
Fig. 7 represents the Sturgeon magnet, A, and the Henry
magnet, B. Around the
Fig. 7.
former (.) are wound the
spirals apart from each
other-the iron core being insulated, and the
copper wire not insulated.
Around the latter, B, the -
wire is insulated with
silk thread, and the coils
are multiplied. This vwas
the magnet invented by
Henry, and which at the time astonished the scientific world.
With the same battery, at least a hundred times more magnetism was produced by Henry's magnet than could have been
obtained by Sturgeon's magnet. The developments were considered at the time of much importance in a scientific point of
view,, and they subsequently furnished the means by which
magneto-electricity, the phenomena of dia-magnetism, and the
magnetic effects on polarized light, were discovered. They gave
rise to the various forms of electro-magnetic machines which
have since distinguished the age. Upon Henry's electromagnet are based the various electro-magnetic telegraphs.
The following may be considered as laws relative to electromagnetism:
1st. The magnetic force developed in the iron is in proportion to the quantity and intensity of the current.
2d. The force, if the current be equal, is independent of
the thickness of the wire or shape of the iron.
3d. Within certain limits, in a continuous coil wound in
layers, like a spool or bobbin of silk, the external turns ar(
as efficacious as those close to the iron.
4th. The total action of the spiral is equal to the sum of the
actions of each turn.
Thus, by increasing the force of the battery so that it;
intensity is augmented twofold, threefold, fourfold, the forc(
of the electro-magnet increases in the same degree. Of course
this force will find its maximum in the conductibility of thi
metal employed in the voltaic circuit.