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.

  1. 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.

  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 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.

  3. 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.)

  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 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.

  5. 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.