The selection of a resistor for a design is most often dictated by the value of resistance needed. The standard unit of measurement for a resistor is the ohm. Resistors come with resistance values that can range from a tenth of an ohm to several hundreds of million ohms. The value of a resistor is often given in units of kilohms and megaohms. A kilohm is equivalent to one thousand ohms, and a megaohm is equivalent to one million ohms. These units are often abbreviated. A resistor with a stated resistance value of 5k has a value of 5,000 ohms. Similarly, a 5M resistor has a resistance value of 5 million ohms.
The most common resistors are cylindrical devices with two leads, one coming from each end of the cylinder. Resistors are generally less than 1/2-inch long and have a diameter that is less than 1/4 inch.
Because the resistance of any material is directly proportional to the length of the material, high-valued resistors are often much longer than a half inch. This is especially true for resistors used in electric utility applications. In these applications, resistors can be greater than a foot in length. Because resistance of any material is inversely proportional to the cross-sectional area of the resistor, the larger the cross-sectional area, the lower its resistance. Resistors that are wider have larger cross-sectional areas and have the capability to carry larger amounts of current; they can operate in high-power applications.
The primary resistor specification is its resistance value. However, the power rating and the resistor's temperature coefficient rating are also often needed for the design of circuits that include resistors. Typical power ratings of resistors range from as low as 1/8 Watt and upwards, to thousands of Watts. In general, the higher the wattage rating, the larger the diameter of the resistor.
The actual resistance of a resistor will vary nonlinearly with temperature. Manufacturers often specify the rate of change of resistance with temperature in parts per million per degree Centigrade.
All resistors, because they are not ideal, that is, perfect resistors, will have other electrical properties associated with them. For many applications these properties are not significant to warrant concern for the design engineers. However, for satellite and other high-frequency communications applications, these properties need to be taken into account.