GUIDELINES ON WIRING ELECTRIC CIRCUITS


In the following experiments, you will be required to wire electric 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.

1.	Voltmeters measure the voltage or potential difference between 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 current 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 create minimal changes in the circuit.

2.	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 positive 
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 affected.

3.	Connecting leads and terminals are considered to have no resistance.  These are drawn as 
lines connecting other circuit elements.  (Actually lead resistances are typically a few 
thousandths of an Ohm, negligible in most cases, but important if other circuit elements have 
comparable resistances.)

4.	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 inversely 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.

6.	Potential differences (voltages) are relative and in a circuit 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 circuit’s 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.

(a)	Currents of the order of a 5mA will result in considerable muscular reaction (shock).
	(b)	Currents of the order of 10mA are capable of paralyzing some muscles (preventing you 
from releasing a “hot” wire).
	(c)	Currents of the order of 40mA will contract respiratory muscles (prevent breathing).
	(d)	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).  




















	GWEC-2