92 INTRODUCTION TO GENERAL SCIENCE 



a. Fill the beaker with a saturated solution of copper sul- 

 phate, attach one copper strip by a wire to the positive ter- 

 minal of the source of electricity, and insert it in the solution. 

 Sandpaper the other copper strip and dip it into melted paraf- 

 fin. When the paraffin becomes set, scratch your initials, 

 or make some design, in the wax, taking care to cut into the 

 copper. Dip the strip in 10 per cent nitric acid, and then put 

 it immediately into the copper sulphate solution. Connect 

 it with the negative terminal of the source of electricity, using 

 some of the German silver wire. The latter regulates the 

 flow of electricity on account of its high resistance. If bubbles 

 collect on the engraving, too much current is passing, and 

 more German silver should be added to the circuit. In half 

 an hour the initials should be raised enough to be visible. 

 Melt off the wax, and clean with benzine. If the strip of 

 copper is heated, it will become oxidized, and then if the en- 

 graving or raised letters are sandpapered, they will stand out 

 in contrast. 



Experiment 31. The Storage Cell. 



Apparatus : Three dry cells, electric bell, two strips of lead 

 I" x 5", beaker 250 c.c., wires. 



Materials : Dilute sulphuric acid 10 per cent. 



a. Connect the three dry cells in series, carbon of one cell 

 with zinc of the next cell, and connect the free ends to the 

 two lead strips. Place the strips in the beaker so that they 

 do not touch, and fill the beaker nearly full of dilute sulphuric 

 acid. Allow the current to pass for two or three minutes. 

 Note the bubbles. They are hydrogen and oxygen. Dis- 

 connect wires from dry cell, and connect them to the bell. It 

 should ring for a few seconds. Why does the bell stop ring- 

 ing ? Charge the storage cell again, but in the opposite direc- 



