GUIDELINES ON WIRING ELECTRIC CIRCUITS
In the following experiments, you will be required to wire elec-
tric circuits. In order to ensure your safety and avoid damage
to equipment and components, it is necessary that extreme caution
be used in hooking the devices together.
There are common rules and conventions which must be followed in
wiring or drawing a schematic diagram of a circuit composed of
electrical components such as power sources, meters for measuring
current or voltage, etc. The following is a listing which should
be sufficient for the experiments scheduled for the next several
weeks.
- Voltmeters measure the voltage or potential difference be-
tween two points in a circuit; that is, the voltage represents
the amount of work per unit charge (Joules / Coul) done by the
electric field as the charge passes between the two points. A
voltmeter is placed in parallel with the path over which the
voltage is measured. An ideal voltmeter will not affect the cur-
rent or voltage of the circuit being studied. For an actual
voltmeter, it is necessary that the current that does in fact
leave the circuit under study and enter the voltmeter be large
enough for the operation of the circuit, but small enough to cre-
ate minimal changes in the circuit.
- An ammeter measures the amount of charge per unit time (Coul
/ s) that passes a point in a circuit. This is the current and
is usually expressed in amperes rather than the equivalent Coul /
sec. An ammeter must be placed in series to correctly determine
the current at a point. That is, all the charges in the current
must pass through the ammeter to give an accurate value for the
current. However, this in turn requires that the resistance of
the ammeter be kept to a minimum to avoid affecting the current
in the circuit. The convention for current is to consider posi-
tive charges moving from a (+) or high potential to (-) or low
potential as required by the effect of an electric field. In
metals, the current is actually composed of electrons which move
in the opposite direction, but the circuit analysis is not af-
fected.
- Connecting leads and terminals are considered to have no re-
sistance. These are drawn as lines connecting other circuit ele-
ments. (Actually lead resistances are typically a few thou-
sandths of an Ohm, negligible in most cases, but important if
other circuit elements have comparable resistances.)
- An ohmmeter is a self-contained ammeter with a fixed voltage
source. An ohmmeter is connected to an isolated element whose
resistance is to be measured, the current that is measured is in-
versely proportional to the resistance. The dial is calibrated
to read the ratio of voltage to current.
WARNING: All electrical meters, resistors, and other devices
have limits for the current and / or voltage they can survive.
In the case of excessive current, heating effects can cause either
excess forces on meter movements or melting. Excess voltage
can cause damage by creating higher currents or by causing rupture
or breakdown under internal electrical forces. Before using
any meter or circuit element, be sure you are aware of any current
or voltage limitations. Ask the instructor, read the label
or catalog description and make a calculation...BEFORE using. In
this laboratory many devices are protected by fuses. Do not rely
on them, however, and please notify the instructor of any burned
out fuses.
- Potential differences (voltages) are relative and in a circuit
cuit are measured relative to some point called ground. The
earth, being a vast reservoir of electrons, cannot have its potential
changed and may be connected through the house ground
wire (or water pipes or any good electrical connection to the
earth) to the point we wish to call ground in the circuit. In
this way the potential of all points in the circuit are known
definitely relative to ground. There are many situations where
the circuits potential floats with respect to the earth until it
is attached to or tied to earth ground. Floating grounds are
usually avoided because of obvious safety problems.
Finally, a word of caution. Electrocution is a serious hazard
and care must be taken to avoid current flow through the human
body. Here are some interesting numbers for you to consider.
- Currents of the order of a 5mA will result in considerable
muscular reaction (shock).
- Currents of the order of
10mA are capable of paralyzing some muscles (preventing you from
releasing a hot wire).
- Currents of the order of 40mA
will contract respiratory muscles (prevent breathing).
- Currents of the order of 100mA for just a few seconds
can result in ventricular fibrillation (rapid, uncoordinated
movements of the heart muscle which prevent it from pumping
blood).