718 CONSTRUCTION OF ELECTRO-MAGNETIC ENGINE. 



face of g and h move smoothly, the gap between g nnd h on each 

 side must be filled up by a small piece of wood, which is cut of the 

 exact width of the gap and pushed into it ; after it is firmly fixed, as 

 much is taken off from its thickness as will make its upper surface 

 exactly flush with that of g and h. After the apparatus has been 

 used several times, a conducting layer of metallic dust is generally 

 rubbed off and deposited in the path of the wires ; this would 

 obviously render the apparatus useless, and the pieces of wood must 

 therefore be replaced by others as soon as such a deposition of metal 

 is perceived. To secure the wooden pieces better it is advisable 

 to file the edges of the metallic projections somewhat slanting, so as 

 to make the space between them wider below, where they are in 

 contact with the wooden suppcrt, than above. 



The ends carried downwards from the spiral round c d must just 

 touch the wood between the projections when c d is in a line with a &, 

 and in every other position they must be in proper contact with the 

 surface of g and h respectively, but not press upon the metal, for 

 such a pressure would considerably increase the friction, and the 

 motion of the apparatus would thus be rendered quite impossible ; 

 on the other hand, if their contact with the metallic surface is any- 

 where imperfect no current can flow through the apparatus, and 

 therefore no motion can be produced. 



The strength of the current required for producing motion de- 

 pends entirely on the nicety with which the apparatus is constructed 

 For an apparatus which is very carefully and neatly made a single 

 Grove or Bunsen cell is sufficient ; two cells will give motion to an 

 apparatus which has been less carefully constructed. 



The most important application of electro-magnetism 

 is electric telegraphy, that is, the transmission of signals 

 to considerable distances. The application rests on 

 the great velocity with which a galvanic current is 

 propagated in a conducting wire, and on the fact that 

 the attraction of an electro-magnet takes place at the 

 instant of closing the circuit, and ceases again when the 

 current is interrupted. 



The velocity of the galvanic current in a telegraph 

 cable is considerably less than that given above for the 

 discharge of a Leyderi jar; nevertheless, currents trans- 



