Read the text. Certain metals, such as silver and copper, have many free electrons flying aimlessly, at high rates of speed

Resistance

Certain metals, such as silver and copper, have many free electrons flying aimlessly, at high rates of speed, at all times, through the spaces between the atoms of the material. Other metals, such as nickel and iron, have fewer free electrons in motion. Other materials, such as glass, rubber, porcelain, mica, quartz, etc., have practically no interatom free-electron movement.

When an e.m.f. is applied across opposite ends of a copper wire, many free electrons progress along the wire and relatively high current results. Copper is a very good conductor of electric current. When the same e.m.f. is applied across an iron wire of equivalent size, only about one-sixth as much current flows. Iron may be considered a fair conductor. When the same e.m.f. is applied across a length of rubber or glass, no electron drift results. These materials are insulators. Insulators are used between conductors when it is desired to prevent electric current from flowing between them.

Silver is the best conductor, and glass is one of the best insulators. Between these two extremes there are many materials of intermediate conducting ability. While such materials can be catalogued as to their conducting ability, it is more usual to think of them by their resisting ability. Glass completely resists the flow of current. Iron resists much less. Silver has the least resistance to current flow.

The resistance a wire or other conducting material will offer to a current depends on four physical factors:

1. The type of material from which it is made (silver, iron, etc.).

2. The length (the longer, the more of resistance).

3. Cross-section area of the conductor (the more area, the less resistance).

4. Temperature (the warmer, the more resistance, except for carbon).

The unit of measurement of resistance is the ohm. For practical purposes an ohm may be considered to be the resistance of a round copper wire, 0.001 in. diameter, 0.88 in. long, al 32° Fahrenheit (J2¹ F). It is common practice to use the Creek letter omega (Ω) to indicate the word ohm(s) in problems and on diagrams where resistance values are given.

In the electric circuit the voltmeter reading divided by the reading is always a constant called the resistance.

The current in amperes is equal to the potential difference in volts divided by the resistance in ohms.

This statement is known as Ohm's law and is the basis for a large portion of electrical circuit theory. It may be expressed in the three forms below:

, , .

Where is current in amperes, is potential difference in volts and is resistance in ohms.