PROBLEMS AND EXERCISES. 431 



of heat is 42 x 10. From these figures calculate the electro- 

 motive force of a Darnell's cell. 



Ans. 113913656 C.G.S units, or 

 1-13913656 volt. 



3. Explain the operation of charging a secondary battery. 

 What are the chemical actions which go on during charging and 

 during discharging ? 



4. Most liquids which conduct electricity are decomposed 

 (except the melted metals) in the act of conducting. How do 

 you account for the fact observed by Faraday that the amount 

 of matter transferred through the liquid and deposited on the 

 electrodes is proportional to the amount of electricity trans- 

 ferred through the liquid ? 



5. Describe the process for multiplying by electricity copies 

 of engravings on wood-blocks. 



6. How would you make arrangements for silvering spoons 

 of nickel-bronze by electro-deposition ? 



QUESTIONS ON CHAPTER XII. 



1 . Sketch an arrangement by which a single line of wire can 

 be used by an operator at either end to signal to the other ; the 

 condition of working being that whenever you are not sending 

 a" message yourself your instrument shall be in circuit with the 

 line wire, and out ^circuit with the battery at your own end. 



2. What advantages has the Morse instrument over the 

 needle instruments introduced into telegraphy by Cooke and 

 Wheatstone ? 



3. Explain the use and construction of a relay. 



4. It is desirable in certain cases (diplex and quadruplex 

 signalling) to arrange telegraphic instruments so that they will 

 respond only to currents which come in one direction through 

 the line. How can this be done ? 



5. A battery is set up at one station. A galvanometer needle 

 at a station eighty miles away is deflected through a certain 

 number of degrees when the wire of its coil makes twelve turns 

 round the needle ; wire of the same quality being used for 

 both line and galvanometer. At 200 miles the same deflection 

 is obtained when twenty -four turns are used in the galvan- 



