Review Questions

1. Define effective resistance. In alternating-current work, why should the old "Ohm's law" definition of resistance be avoided?

2. Explain why the effective resistance of a circuit may differ from the direct-current resistance.

3. A coil has a laminated silicon-steel core. Would you expect the permeability of the core to be exactly the same at 50 cycles as at 10,000 cycles, all other factors being comparable? Explain why.

4. Discuss the nature of the inductance of a coil with a ferromagnetic core.

5. What is the effect of a small air gap on the incremental inductance of a coil with a ferromagnetic core?

6. When phase relations are of importance, why should - j be written before equation 8?

7. On page 53 a somewhat detailed explanation was given of the inductive effects in coils with ferromagnetic cores. What parts of this discussion apply to the mutual inductance of coils with such cores?

8. Would you attempt to use a 10,000-ohm wire-wound resistor at a frequency of 1.0 megacycle? Why? If your answer is negative, what type would you use?

9. In telephone equipment non-inductive resistors are often made by winding the resistance wire on a thin card. Fully explain why this reduces the inductance. What are other advantages of a resistor of this type?

10. Discuss the types of cores used in communication inductors and transformers.

11. Discuss the types of dielectrics used in communication capacitors.

12. What is meant by the "Q" of an inductor and "D" of a capacitor? Are these two terms related?

13. Briefly discuss the magnetic materials used in communication.

14. Briefly discuss the dielectric materials used in communication.

15. Enumerate and briefly discuss the various frequency bands used in communication.

16. On pages 64 and 65, phase resonances were discussed, the term phase being used to prevent confusion with other types of resonance. What other possible types of steady-state electrical resonances are there? Why is the term antiresonance sometimes used?

17. In solving an inductively coupled circuit as explained on page 67, the effect of the secondary reflected into the primary is considered. Explain why it is not necessary to consider the effect of the primary reflected into the secondary.

18. What are the conditions for maximum power transfer in an alternating-current circuit? Why is maximum power transfer and not efficiency often of greatest importance in communication?

19. Why are impedance transformations often made in communication? Name several methods of transforming impedances.

20. In deriving the equations for an alternating-current bridge, why are the in-phase and out-of-phase terms separated?

21. A device called an Inductometer or Variometer is constructed on the principle of the circuit of Fig. 18(a). It is used to furnish the variable inductance for bridge standards. Explain how such a device should be constructed to give variable inductance and constant effective resistance. Could the device have zero inductance?

22. In making bridge measurements of incremental inductance why should a thermocouple not be placed directly in series with the coil under test?

23. In discussing thermocouples, it was stated that the separate-heater type was used for high-frequency measurements. Explain why on the basis of the theory on page 139.

24. Explain why a copper or aluminum shield is effective against a high-frequency

magnetic field. Will such a shield affect the resistance and inductance of an air-cored coil? If so, how?

25. Under what conditions can power ratios in decibels be found from current and voltage ratios? Is the decibel used for purposes other than measuring power ratios? Name several.