To explain further, if a copper wire is connected between the terminals of an electric battery, electrons will move from one molecule to another. As one molecule receives an additional electron, it passes another on to the next molecule. Under the conditions just stated, the current flows out of one terminal of the battery through the wire, and back through the battery in a continuous circuit until the circuit is broken or opened. Besides this relaying of the electrons by the molecules the electrons themselves drift along the wire a few feet a minute. An idea of the number of electrons required to produce even a small value of electric current will be gained from the fact that it is necessary for billions of electrons to be in motion through a small electric lamp every second to light it.

[In the early days of the discovery of electricity certain terminals of electric batteries were termed positive and negative and the current was said to flow from the positive (+) to the negative (-) . In view of later discoveries this assumption is incorrect but has persisted nevertheless. The fact that electric currents are said to flow from plus to minus, positive to negative, or high potential to low potential, when as a matter of fact the reverse is true gives rise to a great deal of confusion. It is important that you remember this fact, while current is said to flow from positive to negative, it actually flows from negative to positive.]

14. Ohm's Law.- In some substances the electrons are so strongly attracted to their corresponding protons that it is impossible to move them by means of the potentials we are using. These substances are known as "non-conductors." In various stages between what are termed "conductors" and "non-conductors" are materials in which the electrons are comparatively hard to move, but which will carry a current depending upon the amount of electrical pressure applied. All conductors offer fairly low "resistance" to the flow of electric current. Metals as a rule have comparatively low resistance, while nonmetallic substances such as porcelain, wood, glass, etc., offer a very high resistance to the flow of electric current.

The resistance offered by a wire is in proportion to its length and cross-section. That is, a wire of a certain material which is twice as long as another wire of the same diameter and material will have twice the resistance. Two wires of the same material and of the same length will offer different resistances to the flow of current if the areas of their cross-sections differ. If one wire has one half the area of cross-section as the other, it will offer twice the resistance to the flow of the current.

The flow of an electric current in an electrical circuit is very similar in its action to the flow of water in a water system. To have a flow of either water or electricity, it is absolutely necessary to have first a pressure. Without pressure there is never any flow. In electrical circuits the pressure is called "potential," and it is measured in units called "volts." Since the unit of potential is called a volt, potential itself is quite often called "voltage." If a pressure is applied to water in an open pipe line by means of a pump, the amount of water which will flow through the pipe depends on two things, first, the pressure; and second, the size of the pipe, which determines its resistance to the flow of water through it. If a certain pressure is applied to the pipe line, a certain flow of water will result. If this pressure is doubled, the rate of flow will be doubled; and, on the other hand, if the pressure is cut in half, the rate of flow will be halved. In like manner, if the size of the pipe is doubled the rate of flow will be doubled; and if the size of the pipe is decreased, the water will flow at a decreased rate in direct proportion to the decrease in the size of the pipe.

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