Chapter 7 Sources of Electricity 119 Th is indicates that an electromotive force is present. Th e output voltage of a thermocouple can be strengthened and used to work large motors, valves, controls, and recording devices. Commercial types of thermocouples employ various kinds of dissimilar metals and alloys, such as nickel-platinum, chromel-alumel, and iron-constantan. Th ese unfamiliar names apply to alloys specially developed for thermocouples. Th e combination indicating device, including a meter and a thermocouple, is called a pyrometer. For an instrument that must be sensitive to temperature change, a large number of thermocouples can be joined in series. Such a group is known as a thermopile. 7.2.3 Electrical Energy from Mechanical Pressure Many crystalline substances, such as quartz, tourmaline, and Rochelle salts, have a peculiar characteristic. When a voltage is applied to the surfaces of the crystal, the crystal becomes distorted. Th e opposite is also true. If a mechanical pressure or force is applied to the crystal surface, a voltage is developed. Th e crystal micro- phone, Figure 7-23, is a familiar example of this process. Sound waves striking a diaphragm, which is mechanically linked to the crystal surfaces, cause distortion in the crystal. Th is develops a voltage across its surfaces. Th us, sound waves are converted to the electrical energy. Creating electricity by the mechanical distortion of a crystal is known as the piezoelectric eff ect. Crystals can be cut for particular operating characteristics. In a later chapter, the use of crystals as frequency controls for radio transmitters will be discussed. 7.2.4 Fuel Cells A fuel cell is constructed much like a battery cell. Two metallic electrodes (anode and cathode) are designed to allow hydrogen and oxygen gases to combine with the electrolyte of potassium hydroxide (KOH). See Figure 7-24. Th e two metallic electrodes are not part of the chemical reaction, but rather a means to allow the gases to combine with the electrolyte. Once the gases combine with the electrolyte, ionization occurs. Th e electrode attached to the oxygen line develops a positive potential, while the electrode connected to the hydrogen line develops a negative potential. Th e chemical reaction in the fuel cell takes Goodheart-Willcox Publisher Timothy Hodgkinson/Shutterstock.com Figure 7-20. Schematic symbol and an example of a photoresistive cell. Goodheart-Willcox Publisher Figure 7-21. Small thermocouple. 0.0 V OFF A COM V Ω A V Ω Thermocouple symbol Iron and copper wires Match Meter Goodheart-Willcox Publisher Figure 7-22. The basic principle of the thermocouple can be demonstrated by heating two dissimilar wires that have been twisted together. Diaphragm Sound waves Crystal Electrical waves Output Goodheart-Willcox Publisher Figure 7-23. A crystal microphone converts sound waves to electrical energy. Copyright Goodheart-Willcox Co., Inc.