REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 143—225
1. Describe an electromagnet and its function in telephony.
2. Sketch an iron-clad electromagnet.
3. What is a differential electromagnet? Sketch and describe one type.
4. State the desirable characteristics of good enamel insulation for magnet wire.
5. If you have a coil of No. 23 double cotton B. and S. wire of 115 ohms resistance and you have to rewind it for 1,070 ohms resistance with double cotton wire, what number of wire would you take? Show calculation.
NOTE. No. 23 d. c wire has res. 1.772 ohms per cubic inch; for the core, 115 ohms. There are required in the coil 1,070 ohms, that is, 9.3 times as much. 1.772 x 9.3 = 16.47 ohms, which must be the resistance per cu. in. This resistance gives, according to Table IV, No. 29 wire.
6. What is an impedance coil? State how it differs from an electromagnet coil.
7. Describe the different kinds of impedance coils.