#### ElectroMaster

##### Administrator

The required voltage neccesary to force current through an electric circuit may be obtained from many sources. Some of these sources may be a battery or a generator. There are many types of both, a flashlight battery has only one cell which supplies 1.5 volts. If you place more than one end to end the voltage is multiplied by the number of batteries. So if you have a two battery flashlight, the voltage is 3 volts, a 3 cell the voltage is 4.5 volts and so on. Some batteries have more than one cell to the battery, your car battery for instance is probably a 12 volt battery, meaning it has 8 cells of 1.5 volts each.

There are many types of generators also, from the thermocouple which is two dissimilar pieces of metal connected together, to the power plant size generator that supplies voltage to your home and office.

**Units of voltage**

Electrical pressure is needed to move electrons through a conductor to cause current to flow. Such pressure is known as electromotive force and is abbreviatedemf. The unit of emf is the volt named after Alessandro Volta (1745 - 1827) the italian researcher who first built a cell which provided electromotive force and which was the forerunner of our modern battery. Either V or E designates voltage (electric pressure), and sometimes the word Potential is also used. All these terms have the same meaning. A volt is the quantity of electromotive force that will cause one ampere of current to flow through one ohm of resistance.

The source of electrons, from a battery or othe electric generating devicec, is referred to as the negative terminal or negative section of the unit. The terminal toward which the electrons flow is designated as the positive terminal. Instead of referring to the two different types of potentials as charges, the term Polarity is used. Thus, a flashlight cell may have a potential (emf) of 1.5 volts, with one terminal having a negative polarity and the other terminal a positive polarity.

In practicle electrical and electronic applications voltages of a fractional value will frequently be encountered, as well as voltages having values up in the thousands, depending on the amount of electric pressure necessary to force current flow through the resistances encountered in the various circuits and devices. Fractional unit values of voltage are often in terms of a millivolt (one thousanth of a volt) or microvolt (one millionth of a volt). Thus 0.001-V can be expressed as 1 millivolt (1-mV) while 0.00003-V can be expressed as 30 microvolt. High voltages are often designated as kilivolts(kV),to indicate thousands of volts. Thus 10 kV indicates 10 thousand volts.

**Current and Conductivity**

A current flow path is provided by wires or other metals and thus form conductors of electricity. The ability of a substance to conduct electric current is termed conductivity. Current can also flow in substances such as liquids, gases, or materials whose composition offers opposition to the flow and limits the amount of current to definite quantities. The unit of electric current is known as the Ampere, named after Andre' Ampere (1775 - 1836) the famous French expermenter and scientist. One ampere of current represents the exact quantity of electrons that flows past a given point in one second and is equal to one Coulomb. The symbol for current is the capitol letter I for intensity or the symbol A for ampere.

All substances do not provide the same degree of conductivity since the number of free electrons present depends on the atomic structure of the substance. Various metals offer different opposition to current flow, with some providing good conductivity and others opposing current flow to a considerable degree. The opposition of a substance to the flow of current is known as Resistance and the unit of measurement for such resistance is termed an OHM, in honor of George Ohm (1787 - 1854) the German professor who formulated the basic law relating to current flow and resistance known as Ohm's Law. The symbol for resistance is the capitol letter R and the symbol for ohms is the greek Omega symbol.

The measure of how well a substance will permit current flow is known as Conductance. Because conductance is functionally opposite to resistance, it is the reciprocal of resistance and is therfore equal to the numeral one divided by the value of resistance, as expressed by the formula I/R. Thus, if a particular resistance is 1000 ohms the conductance is one-onethousanth or 0.001. Because conductance is the opposite of resistance, the unit for conductance is expressed as the word ohm spelled backwards which is MHO. The conductance of the example is 0.001 Mho. A fractional measurement of mho is the micromho. This is one-millionth of a mho. The symbol for mho is or conductance is G.