DYNAMO 



143 



Battery 



FIG. 134. 



The copper plate forming the -f- electrode (pole) 

 wears away, but an equal amount of copper is de- 

 posited on the impression of the type (the elec- 

 trode). In this way a copper film is formed which 

 is the exact image of the type. 



This is the way in which electrotype plates of the pages of books are 

 made (Fig. 134). An impression of the type is prepared in wax, this is 

 covered with graphite (cf. 119) to make it a conductor, and the wax 

 mould is then made the 



Copper _ Copper 



sulphate solution 4. 



m. 



receiving pole in a cop- 

 per sulphate " electronic 

 bath." The copper de- 

 posit is an exact copy of 

 the type. It is, however, 

 very thin; hence it is 

 strengthened, at the back, 

 by a filling of melted 

 metal. The original type 

 is free to be used over and 

 over again in the setting 

 up of other pages of the 

 book. 



Silver-plating (Fig. 135) is carried out in a similar way. The solu- 

 tion contains a silver compound, the pole that wastes away is of silver, 

 and the object to be plated is made the receiving pole. 



160. Dynamo. On a small scale, currents may be 

 produced by batteries (cf. 150); but for larger uses men 

 need more economical and more powerful generators. 



These are commonly known 

 as dynamos. 



We have learned that when 

 a t>ar of soft iron, or a bun- 

 dle of soft-iron wires, is put 

 FIO. 135. . .; 



siiver- P iating. into the field of a coil carry- 



ing a current, the soft iron 



becomes magnetized (cf. 154) . Equally important is the 

 fact that when a magnet is introduced into, or removed 

 from, a coil of insulated wire, a current is produced 



