118 Electricity & Electronics 0.5 volts, the cells can be connected into arrays. Arrays consist of many cells connected in series and parallel to increase the voltage and current capabilities to a level suffi cient to power lights and equipment. One application of power supplied by photovoltaic cells is power for a residential home. Today, solar cells are commonly used to power homes and heat swimming pools. Th e cells can also supply the complete power needs for items such as calculators, watches, satellites, and cars. Solar cells provide clean energy and do not require running power lines underground. See Figure 7-18. Figure 7-19 shows a solar array for providing power. Photoelectric Control Th e same principle of generating electricity from light is used to produce a photoelectric control device called a photo- resistive cell. Photoresistive cells are light-sensitive resistors. Instead of providing a direct supply of electricity, this device is used to vary the amount of current that can pass through it, much like a variable resistor would do. Th e photoresistive cell increases circuit resistance when there is no light. It decreases resistance when there is light. Th e symbol for a photoresistive cell is shown in Figure 7-20. Photoresistive cells are often used in streetlights. Th ese cells will turn the streetlight off during the daylight to save energy when lights are not needed. 7.2.2 Electrical Energy from Heat A device used to indicate and control the heat of electric ovens and furnaces is shown in Figure 7-21. Th is device is called a thermocouple. When two dissimilar metals in contact with each other are heated, a potential diff erence develops between the metals. In the demonstration of Figure 7-22, an iron wire and a copper wire are twisted tightly together. Th eir ends are connected to a sensitive meter. When the fl ame of a lit match heats the twisted joint, a reading on the meter can be observed. Cell Module Panel Array Goodheart-Willcox Publisher Figure 7-19. A solar array consists of many solar cells. Martyn Jandula/Shutterstock.com Figure 7-18. A solar array for recharging an electric vehicle. Copyright Goodheart-Willcox Co., Inc.