In the United States the basic unit of liquid measure is the GALLON, which has a volume of 231 cu in. or 0.13 cu ft. The gallon is subdivided into smaller units as follows:

Units larger than the gallon in liquid measure are as follows:

For petroleum products the standard barrel contains 42 gallons.

In the metric system the basic unit of liquid measure is the LITER, equal in volume to a cubic decimeter, or about 61 cu in. There are 3.785 liters in a U.S. gallon.

Following the usual metric system of nomen-clature, subdivisions and multiples of the liter are as follows:

In an electrical circuit there is a flow of electrons, roughly similar to the flow of water in a water pipe. The flow is occasioned by the production, at a generating station, battery, or other source, of an ELECTROMOTIVE FORCE (E), roughly similar to the "head" of water in a water system. The size of the electromotive force is measured in units called VOLTS.

The rate of flow of the electrons through the circuit is called the CURRENT (I). Current is measured in units called AMPERES.

The usual conductor for transporting a flow of electrons through a circuit is wire. Generally speaking, the smaller the diameter of the wire, the more will be the RESISTANCE (R) to the flow, and the larger the diameter, the less the resistance. Resistance is measured in units called OHMS. The definitions of the units volt, ampere, and ohm are as follows:

1 volt - Electromotive force required to send a current of 1 ampere through a system in which the resistance measures 1 ohm.

1 ohm  - Resistance offered by a system in which the electromotive force is 1 volt and the current, 1 ampere.

The ohm is named for Georg Simon Ohm, a German scientist and early electrical pioneer, who discovered that there is a constant relationship between the electromotive force (E), the current (I), and the resistance (R) in any electrical circuit. This relationship is expressed in "Ohms law" as follows:

From the basic law it follows that

From Ohms law you can (1) determine any one of the three values when you know the other two and (2) determine what happens in the circuit when a value is varied.

Suppose, for example, that the resistance (R) is increased, while the electromotive force (E) remains the same. It is obvious that the current (I) must drop proportionately. To avoid a drop in the current, it would be necessary to increase the electromotive force proportionately.

When an electrical circuit is open (that is, when there is a break in the circuit, such as an open switch), there is no flow of electrons through the circuit. When the circuit is closed, however, the current will begin to flow. With a constant electromotive force (E), the rate at which the current (1) flows will depend on the size of the resistance (R). The size of the resistance will increase with the number of electrical devices (such as lights, motors, and the like) that are placed on the circuit, and the amount of POWER each of these consumes.

Power may be defined as "electrical work per unit of time. " James Watt, another early pioneer in the electrical field, discovered that there is a constant relationship between the electromotive force (E), the current (I), and the power consumption (P) in a circuit. This relationship is expressed in the formula P = IE, from which it follows that

Power is measured in units called WATTS, a watt being defined as the work done in 1 second when 1 ampere flows under an electromotive force of 1 volt.

Suppose, now, that you have a 110-volt circuit in your home. The constant E of this circuit, then, is 110 volts. In the circuit there is probably a 15-ampere fuse. A fuse is a device that will open the circuit by "burning out" if the current in the circuit exceeds 15 amperes. The reason for the existence of the fuse is the fact that the wiring in the circuit is designed to stand safely a maximum current of 15 amperes. A current in excess of this amount would cause the wiring to become red hot, eventually to "burn out, " and perhaps to start an electrical fire. Suppose you light a 60-watt bulb on this circuit. Your E is 110 volts. By the formula

you know that the current in the circuit with the 60-watt bulb on is

or about 0.54 amperes, which is well within the margin of safety of 15 amperes. Dividing 15 amperes by 0.54 amperes you find that this fuse will protect a 27-lamp circuit.

But suppose now that you place on the same one-lamp circuit an electric toaster taking about 1,500 watts (electrical devices are usually marked with the number of watts they consume) and an electrical clothes dryer taking about 1,200 watts. The total P is now 60 + 1,500 + 1,200, or 2,760 watts. The current will now be

or 25 amperes. Theoretically, before it reaches this point, the 15-ampere fuse will burn out and open the circuit.