Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGMagnets may be found in the natural statein the form of an iron oxide, but the majorityare produced by artificial means. Artificialmagnets may be either permanent magnets ortemporary magnets, depending upon their abilityto retain magnetic strength after the magnetizingforce has been removed.Permanent magnets are bars of hardenedsteel or other alloy which have been permanentlymagnetized. Permanent magnets are usedextensively in electrical instruments, meters,telephone receivers, and magnetos.Electromagnets are temporary magnetscomposed of soft- iron cores around which arewound coils of insulated wire. Electromagnetsare used in electric motors, generators, andtransformers. When an electric current flowsthrough the coil, the core becomes magnetized.Magnetism is a field of force exerted inspace. A magnetic field consisting of imaginarylines along which the magnetic force acts surroundseach magnet. A visual representation ofa magnetic field can be obtained by placing aplate of glass over a magnet and sprinkling ironfilings onto the glass. The filings arrange themselvesin a pattern of definite paths between thepoles, along the magnetic lines of force, asshown in figure 20-2.41.4Figure 20-2.— Magnetic field pattern around amagnet.Magnetic flux is the entire quantity of linesin a magnetic field, with gauss being the unitmeasurement of its density. One gauss is equalto one line of force per square centimeter ofmagnetic field.PRODUCING A VOLTAGEThere are six commonly used methods ofproducing a voltage. Magnetism and chemicalaction are the two methods most commonly usedaboard ship; hence the present discussion islimited to these two methods. It should be noted,however, that a voltage can also be produced byfriction, pressure, light,Voltage Produced ByChemical Actionand heat.^Chemical energy is transformed into electricalenergy within the cells of a battery. Shipboarduses of electricity from this sourceInclude power supply for emergency lighting(with dry cell batteries) and the starting of smallengines (with wet cell batteries).The most common dry cell battery consistsof a cylindrical zinc container, a carbon electrode,and an electrolyte of ammonium chlorideand water in paste form. The zinc container isthe negative electrode of the cell; it is lined witha nonconducting material to insulate it from theelectrolyte. When a circuit is formed, the currentflows from the negative zinc electrode tothe positive carbon electrode.In a common wet cell storage battery, theelectrodes and the electroljrte are altered bythe chemical action that takes place when thecell delivers current. Such a battery may berestored to its original condition by forcing anelectric current through it in the opposite directionto that of discharge.The most common wet cell storage batteryin use is the lead-acid battery having an emf of2,2 volts per cell. In the fully charged state, thepositive plates are pure lead peroxide and thenegative plates are pure lead immersed in adilute sulfuric acid electrolyte.When a circuit is formed, the chemical actionbetween the ionized electrolyte and dissimilarmetal plates converts chemical energy to electricalenergy. As the storage battery discharges,the sulfuric acid is depleted by being graduallyconverted to water, while both positive andnegative plates are converted to lead sulfate.This chemical reaction is represented by thefollowing equation, the reversibility of which isdependent upon electrical energy being addedduring the charging cycle.One device for producing a voltage by heat is thethermocouple, discussed in chapter 7 of this text.494