22 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



5. Find the time required for 10 amperes of current to gener- 

 ate 2 cubic feet of hydrogen and i cubic foot of oxygen, 

 both gases being measured at 730 millimeter pressure and at a 

 temperature of 20 C. ; 22 millimeters of the pressure in each 

 case being due to the water vapor which is present. Ans. 11.8 

 hours. 



Note. The amount of hydrogen or oxygen generated in one second by one 

 ampere may be found from the electrochemical equivalent of silver and the atomic 

 weights of hydrogen, oxygen and silver, or the data given in the note to problem 6 

 may be used. 



6. A current which produces a steady reading of 5 amperes on 

 an ammeter generates 186.6 cubic centimeters of a mixture of 

 oxygen and hydrogen in 3 minutes, the mixed gases being meas- 

 ured at a net pressure of 7 1 o millimeters and at a temperature of 

 25 C. Find the error of the ammeter reading. Ans. o. I 

 ampere too low. 



Note. By net pressure in this problem is meant the pressure due to the gas alone 

 after correction has been made for the part of the pressure which is due to the water 

 vapor that is present. 



The water coulombmeter is frequently used for quickly standardizing an ammeter, 

 and it is convenient to note that one ampere in one second generates o. 174 cubic centi- 

 meter of mixed hydrogen and oxygen, the gases being measured dry, at 760 milli- 

 meters pressure, and at a temperature of o C. 



7. A voltaic cell which is free from local action gives a current 

 of i . 5 amperes for 50 hours. Calculate the number of grams of 

 zinc consumed. Ans. 91.5 grams. 



Note. The zinc consumed in a voltaic cell by voltaic action is equal to the amount 

 of zinc that would be deposited in an electrolytic cell by the current which the cell 

 delivers. 



8. A single chromic acid cell consumes 125 grams of zinc 

 during the time that the current from the cell is depositing 25 

 grams of copper from a solution of cupric sulphate (CuSO 4 ). 

 What portion of the zinc is consumed by local action ? Ans. 

 79.4 per cent. 



9. A gravity cell is used to give a steady current of o. I ampere 

 continuously, night and day, for 50 days. During this time 



