INTRODUCTION TO SONAR 



^ 



Note: low-power insulQ'-^d wire-wou"d fcslsiorj hove o>iol leodi ond ore cotor coded 

 bmilor lo the leri-^>ond figure obove eicepr ihor bond A ri double width. 



20.374 

 Figure 10-7. — Color code for axial-lead and 

 radial-lead fixed composition resistors. 



used in our example, a reading of 24K to 26K 

 would be acceptable. 



Low-power, wire-wound resistors have axial 

 leads, and are color coded similar to the regular 

 axial lead resistor, except that band A is double 

 width. 



RESISTANCE 

 ELEMENT 



TERMINAL 

 I 



.WIPER 



Variable Resistors 



Variable resistors are of two general types. 

 They may be rheostats or potentiometers. 



A rheostat normally is used to adjust the cur- 

 rent in a circuit without opening the circuit. Some 

 rheostats, however, are so constructed that the 

 circuit may be opened also. In general, a rheostat 

 has two terminals. One terminal is connected to 

 one end of the resistance element; the other, to 

 the sliding contact. As seen in figure 10-8, the 

 resistance element is circular in shape and is 

 made of resistance wire wound around an insulat- 

 ing form that usually is of a ceramic material. 

 The resistance is decreased by rotating the wiper 

 toward terminal 1. 



To test a rheostat, you first must disconnect 

 it from the circuit. An ohmmeter is used to mea- 

 sure the resistance between terminals 1 and 2 

 (refer to fig. 10-8), with the wiper rotated all the 

 way to terminal 2. This action gives total resis- 

 tance. Moving the wiper slowly back toward ter- 

 minal 1 shows an ever-decreasing resistance 

 until a reading of zero is obtained. If a reading of 

 maximum shows on the meter during this phase 

 of rotating the wiper toward terminal 1, it means 

 the wiper is not making proper contact at that 

 point. 



The potentiometer (often called a "pot") is a 

 control instrument used for varying the amount of 

 voltage applied to an electrical device. The term 



potentiometer customarily refers to any adjusta- 

 ble resistor having three terminals, two of which 

 are connected to the ends of the resistance element 

 and the third to the wiper contact. The potentio- 

 meter is illustrated In figure 10-9. By positioning 

 the sliding contact, any desired voltage within the 

 range of the potentiometer may be selected and 

 used where needed. As a rule, potentiometers are 

 constructed to carry smaller currents than rheo- 

 stats. 



Potentiometers are measured in much the 

 same way as rheostats. When disconnected from 

 the circuit, they may be tested with an ohmmeter 

 by measuring for total resistance between termi- 

 nals 1 and 2. Applying one lead to terminal 3 and 

 the other lead to either terminal 1 or 2 and moving 

 the wiper results in a smoothly increasing or 

 decreasing variation in resistance. 



COILS 



Coils used in electronic circuitry are of many 

 types. Among the types are the inductance coil, 

 which is used to oppose changes in current or fre- 

 cjuency; field and armature coils, as used in motors 

 and generators; and solenoid coils, which are used 

 in electromagnets. 



All types of coils have one characteristic in 

 common: They are made of varying lengths of wire, 

 therefore an ohmmeter may be used to make a 

 continuity test to determine if there are any opens. 



162 



