90 TKEATISE ON ALTERNATING CURRENTS. 



2. If in Question 1 the apparatus had a resistance of 2 ohms, and a self- 

 induction of 0-011 henry, what must be the self-induction of the coil ? 



Answer. 0'02015 henry. 



3. Give a complete design of a choking coil with open magnetic circuit to 

 give a counter E.M.F. of 90 volts, and taking 25 amperes on a 75-frequency 

 circuit. 



4. A choking coil of negligible resistance absorbs 90 volts on a circuit of 

 frequency 100 : what is its counter E.M.F., if the frequency is 75, the current 

 remaining the same ? 



Answer. 67'5 volts. 



5. A choking coil of negligible resistance gives the same counter E.M.F. at 

 frequencies 100 and 80 : what is the ratio of the currents at the two frequencies ? 



Answer, f. 



G. A choking coil has a P.D. between its terminals of 90 volts ; its resistance 

 is 0'5 ohm, and the current is 20 amperes : what is the self-induction of the coil, 

 and what is the difference of phase between the current and P.D., the frequency 

 being 75 ? 



Answer. Self-induction = OO0955 henry 



phase-difference = 83 40'. 



7. If a P.D. represented by 



e = 1000 sin 750* + 100 sin 2250* 



is applied between the terminals of a coil whose resistance is 10 ohms, and self- 

 induction 0-05 henry, find the power given to the circuit. 

 Answer. 3354 watts. 



