# Man's physical universe

CURRENT ELECTRICITY 497

difference in water levels, there is a difference in the electric circuit,

called a potential difference, which is maintained by the electric battery

or the generator which supplies

the energy to maintain a

the wire.

The valve in the water pipe

corresponds to the switch in the

circuit.

The requirements for an electric

current are, therefore, a sustained

difference of potential and a conductor.

Fig. 236. The cell maintains a difference

of potential and supplies the

energy to operate a small motor when

the switch is closed.

Pipes offer resistance which cuts down the flow of water. Electrical

conductors also offer resistance to flow. A small wire will not conduct

so much current as a large wire for equal differences in potential, while

many substances offer a high resistance to the flow of the current.

In making diagrams of electrical circuits, a cell is designated by a

long and a short vertical line parallel to each other.

A straight line

represents a conductor, while a zigzag line represents a resistance.

The resistance of a circuit determines the amount of current that will

flow under a given difference of potential. This is a simple statement of

Ohm's law.

The electric current, the difference of potential, and the resistance

may all be expressed in familiar units:

Current Amperes . (Ampere)

Resistance Ohms . . (Ohm)

Difference of potential . Volts . . (Volta)

Difference of potential is also called voltage. An instrument that

measures voltage is a voltmeter. It corresponds to the pressure gauge

on a water line. The ammeter is the instrument designed to measure

amperes, or current, and corresponds to a water meter (flow-meter

type) which measures the flow of water in gallons per unit of time.

Water meters, which measure the total amount of water flowing,

correspond to coulometers, which measure coulombs. The unit of

electric current, the ampere, corresponds to the flow of one coulomb

per second. A coulomb per second corresponds to the flow of

6.30 X 1018 (6,300,000,000,000,000,000) electrons per second. It is

the amount of current that will cause the deposition of 0.001118 gram

of silver from a solution of silver nitrate in one second. The unit of

resistance, the ohm, is the resistance offered by a conductor which

conducts one ampere under a potential difference of one volt.

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