Chapter 1 Electrical Fundamentals Review 11 Copyright Goodheart-Willcox Co., Inc. called the phase relationship. If alternating current is run through a purely resistive circuit, both voltage and current, although changing, will vary together and are said to be in phase. See Figure 1-13. However, most circuits con- tain components that cause the voltage and current to be out of phase with each other. See Figure 1-14, for instance, where the current lags behind the voltage. That is, the current reaches its peak after the voltage has reached its peak. This is lagging current. Other circuit conditions might cause the current to peak ahead of the voltage, as illustrated in Figure 1-15. 1.3.5 Inductance Inductive devices are another component of ac cir- cuits. These devices create electromagnetic fi elds that decrease the response speed of the circuit. The most basic and useful type of inductive device is a coil. A coil is a conductor formed into a series of loops that may have a solid, mov- able core, Figure 1-16. Electricians working with lighting controls are familiar with the variable inductor (another name for a coil). As illustrated in Figure 1-17, the lamp brightness is controlled by inserting or withdrawing the iron core. The coil has the least inductance when the core is withdrawn and the greatest inductance when the core is fully inserted. Current is greatest when inductance is least. Voltage Current 90° 0° 180° 270° 360° Goodheart-Willcox Publisher Figure 1-13. Voltage and current in phase. Voltage Current Goodheart-Willcox Publisher Figure 1-14. Voltage and current out of phase. The current lags behind the voltage. Voltage Current Goodheart-Willcox Publisher Figure 1-15. Current leading the voltage. Core Coil Goodheart-Willcox Publisher Figure 1-16. A coil with a solid core is one of the simplest types of inductive devices. Alternator Coil Iron core Lamp Goodheart-Willcox Publisher Figure 1-17. The lamp brightness is controlled by insertion of the iron core. When the core is fully inserted, inductance is greatest, current is least, and lamp is dimmest.