Page 58 - Teach Yourself Electricity and Electronics

38 Electrical units

Household voltage, normally about 117 V but sometimes twice that, or about 234 V,
is more than sufficient to kill you if it appears across your chest cavity. Certain devices,
such as automotive spark coils, can produce lethal currents even from the low voltage
(12 V to 14 V) in a car battery.
Consult the American Red Cross or your electrician concerning what kinds of cir-
cuits, procedures and devices are safe, and which kinds aren’t.

Magnetism

Electric currents and magnetic fields are closely related.
Whenever an electric current flows—that is, when charge carriers move—a mag-
netic field accompanies the current. In a straight wire, the magnetic lines of flux
surround the wire in circles, with the wire at the center (Fig. 2-11). Actually, these
aren’t really lines or circles; this is just a convenient way to represent the magnetic field.
You might sometimes hear of a certain number of flux lines per unit cross-sectional
area, such as 100 lines per square centimeter. This is a relative way of talking about the
intensity of the magnetic field.

2-11 Magnetic flux lines around a straight, current-carrying wire. The arrows
indicate current flow.

Magnetic fields can be produced when the atoms of certain materials align them-
selves. Iron is the most common metal that has this property. The iron in the core of the
earth has become aligned to some extent; this is a complex interaction caused by the
rotation of our planet and its motion with respect to the magnetic field of the sun. The
magnetic field surrounding the earth is responsible for various effects, such as the con-
centration of charged particles that you see as the aurora borealis just after a solar
eruption.
When a wire is coiled up, the resulting magnetic flux takes a shape similar to the
flux field surrounding the earth, or the flux field around a bar magnet. Two well-defined
magnetic poles develop, as shown in Fig. 2-12.
The intensity of a magnetic field can be greatly increased by placing a special core
inside of a coil. The core should be of iron or some other material that can be readily