MAGNETISM AND ELECTRICITY 



169 



connection through the key, and explain the use of the 

 key. 



8. Substitute for the bell a sounder from a set of telegraph 

 instruments, and by use of the key cause the sounder to 

 operate.. Describe the course of the current through the 

 sounder, and its manner of operation as the circuit is closed 

 and opened. State (a) when the sounder might operate so 

 feebly as to make it impossible to read its clicks distinctly ; 

 (b) two ways in which to remedy this. 



ELECTRICITY, AND CHEMICAL CHANGES 



1. Fill the tumbler of 

 a simple voltaic cell as 

 shown in the cut two 

 thirds full of water. 

 Have both the zinc and 

 the copper strips of metal 

 scoured bright with sand- 

 paper. To the water 

 add sulphuric acid a few 

 drops at a time till, as 

 the water is stirred with 

 the zinc strip, bubbles 

 form on the metal sur- 

 face somewhat freely. 

 These are not air bubbles, 

 but the gas is hydrogen 

 formed as result of 



chemical change between the acid (H 2 SO 4 ) and the zinc 

 (Zn). It has been found by tests made very many times 



FIG. 75. A simple voltaic cell. 



