Chapter 20. -SHIPBOARD ELECTRICAL SYSTEMS 



DISCHARGING 



Pb + PbOg + 2H2SO 



CHARGING 



_2PbSO^ + 2H2O 



The capacity of a battery is measured in 

 ampere-hours . The capacity is equal to the 

 product of the current (in amperes) and the 

 time (in hours) during which the battery is sup- 

 plying this current to a given load. The capacity 

 depends upon many factors, the most important 

 of which are (1) the area of the plates in contact 

 with the electrolyte, (2) the quantity and specific 

 gravity of the electrolyte, (3) the general condi- 

 tion of the battery, and (4) the final limiting 

 voltage. 



Voltage Produced by Magnetism 



One of the most useful and widely employed 

 applications of magnets is in the production of 

 vast quantities of electric power from mechani- 

 cal sources. The mechanical power may be pro- 

 vided by a number of different devices, including 

 gasoline engines, diesel engines, water tur- 

 bines, steam turbines, and gas turbines. The 

 final conversion of these energies to electricity 

 is done by generators employing the principle 

 of electromagnetic induction. 



There are three conditions which must exist 

 before a voltage can be produced by electro- 

 magnetic induction. First, we must have a 

 magnetic field; second, a conductor; and third, 

 relative motion between the field and the con- 

 ductor. In accordance with these conditions, 

 when a conductor is moved across a magnetic 

 field so as to cut the lines of force, electrons 

 within the conductor are forced to move; thus 

 a voltage is produced. , 



Producing a voltage by magnetic induction 

 is illustrated in figure 20-3. If the ends of a 

 conductor are connected to a low-reading volt- 

 meter or galvanometer and the conductor is 

 moved rapidly down through a magnetic field, 

 there is a momentary reading on the meter. 

 When the conductor is moved up through the 

 field, the meter deflects in the opposite direc- 

 tion. If the conductor is held stationary and the 

 magnet is moved so that the field cuts across 

 the conductor, the meter is deflected in the same 

 manner as when the conductor was moved and 

 the field was stationary. 



The voltage developed across the conductor 

 terminals by electromagnetic induction is known 

 as an induced emf , and the resulting current that 

 flows is called induced current. The induced 



CONDUCTOR MOVED 

 DOWN 



CONDUCTOR MOVED 

 UP 



CONDUCTOR 

 MOTION 



LEFT-HAND GENERATOR RULE 



12.143 

 Figure 20-3.— Left-hand generator rule. 



emf exists only so long as relative motion 

 occurs between the conductor and the field. 



There is a definite relationship between the 

 direction of flux, the direction of motion of the 

 conductor, and the direction of the induced emf. 

 When two of these directions are known, the 

 third can be found by applying the left-hand rule 

 for generators . To find the direction of the emf 

 induced in a conductor, extend the thumb, the 

 index finger, and the second finger of the left 

 hand at right angles to each other, as shown in 

 figure 20-3. Point the index finger in the direc- 

 tion of the flux (toward the south pole) and the 

 thumb in the direction in which the conductor 

 is moving in respect to the fields. The second 

 finger then points in the direction in which the 

 induced emf will cause the electrons to flow. 



DIRECT-CURRENT CIRCUITS 



An electric circuit is a complete path through 

 which electrons can flow from the negative ter- 

 minal of the voltage source, through the con- 

 necting wires (conductors), through the load, and 

 back to the positive terminal of the voltage 



495 



