54 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



placed in a furnace its resistance is 1,630 ohms. What is the 

 temperature of the furnace ? Ans. 1,531. 



30. A platinum wire which has 254 ohms resistance at 

 o C. has a resistance of 81 ohms when placed in a bath of 

 liquid air. What is the temperature of the liquid air ( ? Ans. 



31. A glow lamp has a resistance of 220 ohms at a tempera- 

 ture of i, 000 C. (a bright red heat). At 20 C. its resistance is 

 277 ohms. What is the mean temperature coefficient of the 

 carbon filament? Ans. 0.00021. 



32. The temperature coefficient of a given metal is 0.004 P er 

 degree centigrade when expressed in terms of the resistance of the 

 metal at o C. Find the temperature coefficient per degree 

 Fahrenheit expressed in terms of the resistance at o F. Ans. 

 0.00239 per degree F. 



Note. Assume a wire of the given metal of which the resistance at o C. is one 

 ohm and calculate its resistance R at 17.78 C. (equals o F. ). The tempera- 

 ture coefficient per degree centigrade expressed in terms of the resistance at 17.78 

 C. is greater than the temperature coefficient per degree centigrade expressed in terms 

 of the resistance at o C. in the ratio of R to unity and this result must be divided 

 by 1.8 to get the coefficient per degree Fahrenheit in terms of the resistance at o F. 



33. Practically all of the energy of the chemical action which 

 takes place in the gravity cells goes to maintain the current pro- 

 duced by the cell. When one gram of powdered zinc is stirred 

 into a solution of copper sulphate 756 calories of heat are gener- 

 ated. Calculate the electromotive force of the Daniell cell. 

 Ans. 1.07 volts. 



Note. Assume the current of one ampere and find the fraction of a gram z of zinc 

 which would be deposited by this current per second. This is the amount of zinc 

 which is consumed per second by voltaic action. Find the number of calories of heat 

 represented by the reaction of z grams of zinc with copper sulphate, and reduce this 

 result to joules. We thus find the number of joules per second developed by the vol- 

 taic action which is produced when one ampere flows through the cell and this is 

 equal to the desired electromotive force in volts. 



34. A fan blower develops between its inlet and outlet a pres- 

 sure-difference of three fourths pound per square inch. When 

 the outlet is open the fan delivers 20 cubic feet of air per second. 



