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The Units of Measurement. Direct and Alternating Currents. Ohm’s Law.

Electric Currents may be divided into two classes known as direct and alternating current. Either one may be measured or qualified by two electrical units called the Ampere and the Volt. The volt may be explained by likening it to the "unit of pressure" of the current, while the ampere measures the unit rate of current flow. For example, in the case of water the voltage corresponds to the pressure in pounds while the amperage would indicate the rate of water flowing.

FIG. 11. Hydraulic Analogy between Voltage and Amperage.

The accompanying sketches show graphically the analogy between the voltage and amperage of an electric current and the pressure and volume of a stream of water. In the first illustration a tank is shown at a high elevation from which a small pipe leads. The voltage or pressure in such a pipe would be high in comparison with that in a pipe leading from a lower tank.

In the second illustration the pipe leading from the tank is much larger than that from the first and consequently the amperage or volume flowing is greater in comparison. From this it may be readily seen that every circuit through which a current is flowing must exhibit both quantities.

The unit of electrical work or energy is the Watt. Seven hundred and forty-six watts constitute an electrical horse-power. The number of watts is indicated by the voltage times the amperage. Thus the amount of energy in a circuit in which 50 amperes at 100 volts pressure are passing is 50 x 100 or 5,000 watts.

The Couloumb represents the quantity of electricity flowing in a circuit-where the rate of flow is one ampere per second.

In order to properly indicate comparative amounts of energy the element of time must also be taken into consideration. One watt passing for one hour is a watt-hour. Seven hundred and forty-six watts passing for one hour or one watt passing for seven hundred and forty-six hours is a horse-power hour.

The instruments used for measuring the amperage and voltage of a circuit are called respectively the ammeter and the voltmeter. That used for registering watt-hours is called the integrating watt-meter.

FIG. 12. Diagram Showing Alternating and Direct Current.

Direct current is current that passes or flows in one direction only. The current of all primary and secondary cells and of certain forms of dynamos is direct.

Alternating current is current that repeatedly reverses its direction of flow. A direct current may be represented by a straight line. An alternating current is shown by a wavy line crossing and recrossing a straight line. The current gradually rises from zero to a maximum and then dies away. It does not stop at this point however, but starts to rise again, this time flowing in a reverse direction. After reaching a maximum it dies away again and the cycle is repeated. From a to c represents a cycle and from a to b an alternation. Alternating currents usually have a frequency of 30, 60 or 120 cycles per second. Sixty is the most common frequency. Many wireless telegraph stations now employ currents having a frequency of 500 cycles.

Ohm’s Law.

Mention has been made above of certain electrical magnitudes, namely, voltage or electromotive force and amperage or strength of current. These bear an important relation in determining a property of an electric circuit called resistance.

No conducting body possesses perfect electrical conductivity, but presents a certain amount of obstruction or resistance to the passage of electricity. The practical unit of resistance is the Ohm. It is represented by the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice, 14.4521 grams in mass, of a constant cross sectional area and of the length of 106.3 centimetres.

The resistance of a conductor is proportional to its length, that is, provided two conductors are made of the same material and of the same diameter and one is twice as long as the other, the resistance of the longer will be twice that of the shorter conductor. The resistance is inversely proportional to the cross sectional area, which is to say that a conductor of smaller cross section has a greater resistance than one of larger section.

The laws of resistance are conveniently expressed by the following formula called Ohm’s Law.

C = E/R

where E=electromotive force in volts.

C=current in amperes.

R=resistance in ohms.

If two factors are known, the third can be found by substitution.