166 



FIRST YEAR SCIENCE 



Fig. 79. 



of copper and a sheet of zinc placed so that they do not 

 touch and put in a dish containing dilute sulphuric acid 

 (Fig. 79). The current developed 

 by this cell is very weak. At the 

 present time dry cells are used for 

 almost all ordinary purposes in which 

 electric batteries are needed. 



The history of the development of 

 our knowledge of primary cells and 

 current electricity is exceedingly in- 

 teresting and important, but it can- 

 not be dwelt upon here. In 1832 an 

 American, Samuel F. B. Morse, invented the commercial 

 telegraph. This was the first step in the wonderful prog- 

 ress that has been made 

 during the last century 

 in communicating rap- 

 idly between distant 

 points. The necessary 

 instruments used in this 

 form of communication 

 are a sounder (Fig. 80) 

 and a key (Fig. 81). Fi S- 80 - 



The sounder is simply an electro-magnet such as was made 

 in Experiment 14, arranged to attract a piece of soft iron 



held at a short distance from it 

 by a spring. When this piece of 

 iron is attracted toward the mag- 

 net, it strikes on another piece of 

 iron, making a click, and so re. 

 Fj gl mains drawn to the magnet as 



long as the circuit is kept closed. 



Thus long and short clicks can be made. Morse arranged 

 a combination of these long and short clicks to represent 



