The circuit in the previous section is not a very practical one. In fact, it
can be quite dangerous to build (directly connecting the poles of a voltage
source together with a single piece of wire). The reason it is dangerous is
because the magnitude of electric current may be very large in such a short
circuit, and the release of energy very dramatic (usually in the form of
heat). Usually, electric circuits are constructed in such a way as to make
practical use of that released energy, in as safe a manner as possible.
One practical and popular use of electric current is for the operation of
electric lighting. The simplest form of electric lamp is a tiny metal
"filament" inside of a clear glass bulb, which glows white-hot
("incandesces") with heat energy when sufficient electric current
passes through it. Like the battery, it has two conductive connection points,
one for electrons to enter and the other for electrons to exit.
Connected to a source of voltage, an electric lamp circuit looks something
As the electrons work their way through the thin metal filament of the lamp,
they encounter more opposition to motion than they typically would in a thick
piece of wire. This opposition to electric current depends on the type of
material, its cross-sectional area, and its temperature. It is technically known
as resistance. (It can be said that conductors have low resistance and
insulators have very high resistance.) This resistance serves to limit the
amount of current through the circuit with a given amount of voltage supplied by
the battery, as compared with the "short circuit" where we had nothing
but a wire joining one end of the voltage source (battery) to the other.
When electrons move against the opposition of resistance,
"friction" is generated. Just like mechanical friction, the friction
produced by electrons flowing against a resistance manifests itself in the form
of heat. The concentrated resistance of a lamp's filament results in a
relatively large amount of heat energy dissipated at that filament. This heat
energy is enough to cause the filament to glow white-hot, producing light,
whereas the wires connecting the lamp to the battery (which have much lower
resistance) hardly even get warm while conducting the same amount of current.
As in the case of the short circuit, if the continuity of the circuit is
broken at any point, electron flow stops throughout the entire circuit. With a
lamp in place, this means that it will stop glowing:
As before, with no flow of electrons, the entire potential (voltage) of the
battery is available across the break, waiting for the opportunity of a
connection to bridge across that break and permit electron flow again. This
condition is known as an open circuit, where a break in the continuity of
the circuit prevents current throughout. All it takes is a single break in
continuity to "open" a circuit. Once any breaks have been connected
once again and the continuity of the circuit re-established, it is known as a closed
What we see here is the basis for switching lamps on and off by remote
switches. Because any break in a circuit's continuity results in current
stopping throughout the entire circuit, we can use a device designed to
intentionally break that continuity (called a switch), mounted at any
convenient location that we can run wires to, to control the flow of electrons
in the circuit:
This is how a switch mounted on the wall of a house can control a lamp that
is mounted down a long hallway, or even in another room, far away from the
switch. The switch itself is constructed of a pair of conductive contacts
(usually made of some kind of metal) forced together by a mechanical lever
actuator or pushbutton. When the contacts touch each other, electrons are able
to flow from one to the other and the circuit's continuity is established; when
the contacts are separated, electron flow from one to the other is prevented by
the insulation of the air between, and the circuit's continuity is broken.
Perhaps the best kind of switch to show for illustration of the basic
principle is the "knife" switch:
A knife switch is nothing more than a conductive lever, free to pivot on a
hinge, coming into physical contact with one or more stationary contact points
which are also conductive. The switch shown in the above illustration is
constructed on a porcelain base (an excellent insulating material), using copper
(an excellent conductor) for the "blade" and contact points. The
handle is plastic to insulate the operator's hand from the conductive blade of
the switch when opening or closing it.
Here is another type of knife switch, with two stationary contacts instead of
The particular knife switch shown here has one "blade" but two
stationary contacts, meaning that it can make or break more than one circuit.
For now this is not terribly important to be aware of, just the basic concept of
what a switch is and how it works.
Knife switches are great for illustrating the basic principle of how a switch
works, but they present distinct safety problems when used in high-power
electric circuits. The exposed conductors in a knife switch make accidental
contact with the circuit a distinct possibility, and any sparking that may occur
between the moving blade and the stationary contact is free to ignite any nearby
flammable materials. Most modern switch designs have their moving conductors and
contact points sealed inside an insulating case in order to mitigate these
hazards. A photograph of a few modern switch types show how the switching
mechanisms are much more concealed than with the knife design:
In keeping with the "open" and "closed" terminology of
circuits, a switch that is making contact from one connection terminal to the
other (example: a knife switch with the blade fully touching the stationary
contact point) provides continuity for electrons to flow through, and is called
a closed switch. Conversely, a switch that is breaking continuity
(example: a knife switch with the blade not touching the stationary
contact point) won't allow electrons to pass through and is called an open
switch. This terminology is often confusing to the new student of electronics,
because the words "open" and "closed" are commonly
understood in the context of a door, where "open" is equated with free
passage and "closed" with blockage. With electrical switches, these
terms have opposite meaning: "open" means no flow while
"closed" means free passage of electrons.
- Resistance is the measure of opposition to electric current.
- A short circuit is an electric circuit offering little or no
resistance to the flow of electrons. Short circuits are dangerous with high
voltage power sources because the high currents encountered can cause large
amounts of heat energy to be released.
- An open circuit is one where the continuity has been broken by an
interruption in the path for electrons to flow.
- A closed circuit is one that is complete, with good continuity
- A device designed to open or close a circuit under controlled conditions
is called a switch.
- The terms "open" and "closed" refer to
switches as well as entire circuits. An open switch is one without
continuity: electrons cannot flow through it. A closed switch is one that
provides a direct (low resistance) path for electrons to flow through.
Lessons In Electric Circuits copyright (C) 2000-2011 Tony R. Kuphaldt,
under the terms and conditions of the Design