PROBLEMS. 45 1 



into the solenoid when a current of 20 amperes flows in the wire. 

 Ans. 8.014 x io 6 dynes. 



NOTE. One pole of the rod is in the intense field inside of the solenoid and is 

 pulled with a force of mdt dynes. The other pole of the rod is remote from the 

 solenoid and the force acting upon it may be neglected. 



19. A current of 0.5 ampere flowing through a glow lamp 

 generates 150 calories of heat in io seconds, (a) Required the 

 resistance of the lamp in ohms. () What power is expended 

 in the lamp ? Express in watts and in horse-power. Ans. (a) 

 252 ohms; (b) 63 watts or 0.0844 horse-power. 



20. A wire having a resistance of 250 ohms is coiled in a 

 vessel containing 2,000 grams of oil of which the specific heat 

 is 0.60. The vessel itself weighs 200 grams and its specific heat 

 is 0.95. A current of 1.5 amperes is passed through the coil of 

 wire. How long will it take to raise the temperature of the oil 

 and the vessel one centigrade degree ? Ans. 9. 1 1 seconds. 



21. The field coil of a dynamo contains 25 pounds of copper 

 (specific heat 0.094), weight of cotton insulation negligible. The 

 resistance of the coil is 100 ohms, (a) At what rate does the 

 temperature of the coil begin to rise when a current of 0.5 am- 

 pere is started in the coil ? (<) How long would it take for the 

 temperature of the coil to rise 20 C. if no heat were given off 

 from the coil by radiation ? Ans. (a) 0.0056 centigrade degrees 

 per second ; (b) 59 minutes, 3 1 seconds. 



22. What is the resistance at 25 C. of one mile of commer- 

 cial copper wire 200 mils in diameter? Ans. 1.426 ohms. 



NOTE. One mil is a thousandth of an inch. One circular mil is the area of a 

 circle one mil in diameter. The area of a circle d mils in diameter is d 2 circular mils. 

 When length of wire is expressed in feet and sectional area in circular mils the numer- 

 ical value of k in equation (12), Chapter I., for commercial copper at 25 C. is about 

 10.8 ohms. 



23. A given spool wound full of copper wire 102 mils in 

 diameter has a resistance of 1 . 5 ohms. An exactly similar spool 

 is wound full of copper wire 204 mils in diameter, what is its re- 

 sistance? Ans. 0.0938 ohm. 



