Experiment 25: Magnetism
238 Chapter 5
BAckground
Joseph Henry
Born in 1797, Joseph Henry was the first to develop and demonstrate powerful
electromagnets. He also originated the concept of “self-inductance,” meaning
the “electrical inertia” that is a property of a coil of wire.
Henry started out as the son of a day laborer in Albany, New York. He worked in
a general store before being apprenticed to a watchmaker, and was interested
in becoming an actor. Friends persuaded him to enroll at the Albany Academy,
where he turned out to have an aptitude for science. In 1826, he was appointed
Professor of Mathematics and Natural Philosophy at the Academy, even though
he was not a college graduate and described himself as being “principally self-
educated.” Michael Faraday was doing similar work in England, but Henry was
unaware of it.
Henry was appointed to Princeton in 1832, where he received $1,000 per year
and a free house. When Morse attempted to patent the telegraph, Henry testi-
fied that he was already aware of its concept, and indeed had rigged a system
on similar principles to signal his wife, at home, when he was working in his
laboratory at the Philosophical Hall.
Henry taught chemistry, astronomy, and architecture, in addition to physical
science, and because science was not divided into strict specialties as it is now,
he investigated phenomena such as phosphorescence, sound, capillary action,
and ballistics. In 1846, he headed the newly founded Smithsonian Institution as
its secretary.Figure 5-17. Joseph Henry was an American experimenter who pioneered the inves-
tigation of electromagnetism. This photograph is archived in Wikimedia Commons.FundAmentAls
Coil schematics and
basics
Check the schematic symbols for
coils in Figure 5-16. Note that if a
coil has an iron core, this is shown
with an extra couple of lines
(sometimes only one line). If it has
a ferrite core, the line is sometimes
shown with dashes.
An iron core will add to the
inductance of a coil, because it
increases the magnetic effect.
A coil in isolation does not gener-
ally have any polarity. You can con-
nect it either way around, but the
magnetic force will be reversed ac-
cordingly (coils that interact with
stuff—such as in transformers and
solenoids—do have polarity).
Perhaps the most widespread
application of coils is in transform-
ers, where alternating current
in one coil induces alternating
current in another, often sharing
the same iron core. If the primary
(input) coil has half as many turns
as the secondary (output) coil, the
voltage will be doubled, at half
the current—assuming hypotheti-
cally that the transformer is 100%
efficient.Figure 5-16. Schematic diagrams
represent coils. At far right is the
older style. The third and fourth
symbols indicate that the coil is
wound around a solid or powdered
magnetic core, respectively.