PRINCIPLES OF NAVAL ENGINEERING 



Magnets may be found in the natural state 

 in the form of an iron oxide, but the majority 

 are produced by artificial means. Artificial 

 magnets may be either permanent magnets or 

 temporary magnets, depending upon their ability 

 to retain magnetic strength after the magnetiz- 

 ing force has been removed. 



Permanent magnets are bars of hardened 

 steel or other alloy which have been perma- 

 nently magnetized. Permanent magnets are used 

 extensively in electrical instruments, meters, 

 telephone receivers, and magnetos. 



Electromagnets are temporary magnets 

 composed of soft- iron cores around which are 

 wound coils of insulated wire. Electromagnets 

 are used in electric motors, generators, and 

 transformers. When an electric current flows 

 through the coil, the core becomes magnetized. 



Magnetism is a field of force exerted in 

 space. A magnetic field consisting of imaginary 

 lines along which the magnetic force acts sur- 

 rounds each magnet. A visual representation of 

 a magnetic field can be obtained by placing a 

 plate of glass over a magnet and sprinkling iron 

 filings onto the glass. The filings arrange them- 

 selves in a pattern of definite paths between the 

 poles, along the magnetic lines of force, as 

 shown in figure 20-2. 





41.4 

 Figure 20-2.— Magnetic field pattern around a 

 magnet. 



Magnetic flux is the entire quantity of lines 

 in a magnetic field, with gauss being the unit 

 measurement of its density. One gauss is equal 

 to one line of force per square centimeter of 

 magnetic field. 



PRODUCING A VOLTAGE 



There are six commonly used methods of 

 producing a voltage. Magnetism and chemical 



action are the two methods most commonly used 

 aboard ship; hence the present discussion is 

 limited to these two methods. It should be noted, 

 however, that a voltage can also be produced by 

 friction, pressure, light, and heat.^ 



Voltage Produced By 

 Chemical Action 



Chemical energy is transformed into elec- 

 trical energy within the cells of a battery. Ship- 

 board uses of electricity from this source 

 Include power supply for emergency lighting 

 (with dry cell batteries) and the starting of small 

 engines (with wet cell batteries). 



The most common dry cell battery consists 

 of a cylindrical zinc container, a carbon elec- 

 trode, and an electrolyte of ammonium chloride 

 and water in paste form. The zinc container is 

 the negative electrode of the cell; it is lined with 

 a nonconducting material to insulate it from the 

 electrolyte. When a circuit is formed, the cur- 

 rent flows from the negative zinc electrode to 

 the positive carbon electrode. 



In a common wet cell storage battery, the 

 electrodes and the electroljrte are altered by 

 the chemical action that takes place when the 

 cell delivers current. Such a battery may be 

 restored to its original condition by forcing an 

 electric current through it in the opposite di- 

 rection to that of discharge. 



The most common wet cell storage battery 

 in use is the lead-acid battery having an emf of 

 2,2 volts per cell. In the fully charged state, the 

 positive plates are pure lead peroxide and the 

 negative plates are pure lead immersed in a 

 dilute sulfuric acid electrolyte. 



When a circuit is formed, the chemical action 

 between the ionized electrolyte and dissimilar 

 metal plates converts chemical energy to elec- 

 trical energy. As the storage battery discharges, 

 the sulfuric acid is depleted by being gradually 

 converted to water, while both positive and 

 negative plates are converted to lead sulfate. 

 This chemical reaction is represented by the 

 following equation, the reversibility of which is 

 dependent upon electrical energy being added 

 during the charging cycle. 



One device for producing a voltage by heat is the 

 thermocouple, discussed in chapter 7 of this text. 



494 



