Chapter 10 — SAFETY; TEST EQUIPMENT; TEST METHODS 



WIPER 



RESISTANCE 

 ELEMENT 



TERMINAL 2 

 TERMINAL 3 



TERMINAL I 



WIPER (TERMINAL 3) 



Figure 10-9. 



33.75(71) 

 Potentiometer. 



or breaks, in the wire. This continuity ciieck is the 

 simplest test for coils. Other tests for measuring 

 the current flow and voltage can be made with vari- 

 ous kinds of test equipment. 



Direct current and alternating current work 

 differently when there is a coil in the circuit. When 

 a circuit containing a coil is energized with direct 

 current, the coil's effect in the circuit is evident 

 only at the instant the circuit is energized or deen- 

 ergized. For instance, when the switch in figure 

 10-10 is placed in position 1, the inductance of coil 

 L causes a delay in the time required for the lamps 

 to attain normal brilliance. After they attain nor- 

 mal brilliance, the inductance has no effect on the 

 circuit so long as the switch remains closed. When 

 the switch is opened, an electric spark jumps 

 across the opening switch contacts. The spark is 

 caused by the collapsing magnetic field cutting the 

 turns of the inductor. 



When the inductive circuit is supplied with al- 

 ternating current, however, the inductor's effect 

 is continuous. For equally applied voltages, the 

 current through the circuit is less when alterna- 

 ting current is applied than when direct current is 

 applied and is allowed to reach its steady state. 



— nov 



~ OC SOURCE 



L 



nov 



A-C SOURCE 



nov LAMPS 



71.95 

 Figure 10-10. — Inductance in a-c andd-c circuits. 



The alternating current is accompanied by an 

 alternating magnetic field around the coil, which 

 cuts through the turns of the coil. This action 

 induces a voltage in the coil that always opposes 

 the changing current. When the switch is in posi- 

 tion 1, the lamps burn brightly on direct current. 

 In position 2, although the effective value of the 

 applied a-c voltage is equal to the d-c value, the 

 lamps burn dimly because of the opposition devel- 

 oped across the inductor. Most of the applied vol- 

 tage appears across coil L, with little remaining 

 for the lamps. 



An electromagnet is another device that uses a 

 coil. The electric bell is one of the most common 

 instruments employing an electromagnet, A sim- 

 ple electric bell is diagramed in figure 10-11. Its 

 operation is explained as follows: 



SCREW CONTACT SOFT- IRON ARMATURE 



HAMMER 

 / 



71.96 

 Figure 10-11. — Electric bell. 



163 



