222 



OUR PHYSICAL WORLD 



which are separated when the hammer is pulled over so as to 

 strike the bell. When the current ceases to flow in the coil, its 

 core ceases to be a magnet, and the iron bar with its attached 

 hammer is drawn back by a spring to its initial position. Thus 

 contact between the points is again established, the core of the 

 coil again becomes a magnet, and the hammer again strikes the 

 bell. This process is repeated much more rapidly than it can be 

 described. The bell therefore rings with a rolling note like that 

 of a drum. The electric buzzer is similarly constructed and 

 operated, but since it has neither bell nor hammer, only a rat- 

 tling noise is produced as the iron bar strikes first the core of the 

 coil and then the post that bears the contact point (Fig. 92). 



utn 



FIG. 92. Diagram of a buzzer, push button, and batteries connected up 

 properly/ 



The rapid increase in the use of telegraphic communication 

 created a great demand for more efficient types of batteries. 

 Since Volta first discovered how to make a battery to produce 

 the so-called galvanic electricity, very many types of batteries 

 have been produced, though the principle of operation is much 

 the same in all. The succession of events that produces the elec- 

 tric current may be described for one or two types of cells. 



When a strip of copper and a piece of carbon, such as an old 

 electric light carbon, are partially immersed in dilute acid at the 

 opposite sides of a tumbler, and their free ends are connected 

 by a wire, a simple battery is made, and an electric current flows 

 through the wire. The copper replaces the hydrogen of the acid, 

 forming copper chloride, CuCl 2 . This in part ionizes, separating 



