PRINCIPLES OF NAVAL ENGINEERING 



and 



Ebc 



^3 = Rr 



10 



= 1. 666 amperes 



According to Kirchhoff's current law, the 

 sum of the currents flowing in the individual 

 parallel branches is equal to the total current. 

 Thus, 



l2 + ^3 = It 



or 



3. 333 + 1. 666 = 5 amperes (approx. ) 



The total current flows through Rl; at point 

 b it divides between the two branches in inverse 

 proportion to the resistance of the branches. 

 Twice as much goes through R2 as through R3 

 because R2 has one-half the resistance of R3. 

 Thus, 3.333 (or two-thirds of 5) amperes flow 

 through R2; and 1.666 (or one-third of 5) am- 

 peres flow through R3. 



WHEATSTONE BRIDGE 



A type of circuit that is widely used for pre- 

 cision measurements of resistance is the Wheat - 

 stone bridge . The circuit diagram of a Wheat- 

 stone bridge is shown in figure 20-9. Rl, R2, 

 and R3 are precision variable resistors, andR^ 

 is the resistor whose unknown value is to be 

 determined. The galvanometer, G, is inserted 

 across terminals b and d to indicate the condi- 

 tion of balance. When the bridge is properly 



balanced, there is no difference in potential 

 across terminals b and d_and the galvanometer 

 deflection, when the switch is closed, will be 

 zero. Should the bridge become unbalanced due 

 to a change in resistance of R^, the difference 

 of potential between terminals b anddwLll cause 

 a deflection in the galvanometer. 



When this type of circuit is used as a com- 

 ponent of a resistance thermometer, Rx is the 

 temperature-sensing element. The resistance of 

 Rjj varies directly with the temperature; thus a 

 change in temperature results in an unbalanced 

 bridge and a deflection of the galvanometer. 



DIRECT-CURRENT GENERATORS 



A d-c generator is a rotating machine that 

 converts mechanical energy into electrical en- 

 ergy. This conversion is accomplished by rotat- 

 ing an armature, which carries conductors, in 

 a magnetic field, thus inducing an emf in the 

 conductors. 



A d-c generator (fig. 20-10) consists essen- 

 tially of a steel frame or yoke containing the 

 pole pieces and field windings; an armature con- 

 sisting of a group of copper conductors mounted 

 in a slotted cylindrical core; a commutator for 

 maintaining the current in one direction through 

 the external circuit; and brushes with brush 

 holders to carry the current from the commu- 

 tator to the external load circuit. 



The frame, in addition to providing mechan- 

 ical support for the pole pieces, serves as a 

 portion of the magnetic circuit in that it pro- 

 vides a path for the magnetic flux between the 

 poles. 



12.251 

 Figure 20-9.— Wheatstone bridge circuit 

 diagram. 



CAP SCREW 

 FOR MOUNTING 

 POLE PIECE 



FIELD POLE 



AND 

 FIELD CORE 



ARMATURE 



COMMUTATOR 



73.161 



Figure 20-10.— A d-c generator. 



500 



