PART II. 



ELEMENTS OF THE SCIENCE AND PRACTICE OF ELECTRIC 

 TELEGRAPHY. 



I. ORIGIN OF THE GALVANIC CURRENT. 



1. Any piece of metal, when partly immersed in a liquid, 

 according to Pfaff's experiments, becomes polarised the 

 part above the liquid being negative, and that in the liquid 

 positive. The strongest polarisations are those set up by 

 zinc and tin in solutions containing free nitric or sul- 

 phuric acids ; therefore zinc and tin are termed the most 

 powerful electro-motors. The polarisation of the metal, a 

 piece of zinc for instance, is communicated to the particles 

 of the liquid which tend to arrange themselves in order 

 according to their component atoms : thus, if the liquid be 

 pure water, each of whose atoms is composed of one atom 

 of hydrogen and one atom of oxygen, these atoms of water 

 will probably so arrange themselves that the oxygen and 

 hydrogen sides will stand alternately with regard to the 

 zinc ; for, according to Grotthuss, the atoms of oxygen and 

 hydrogen composing the water are held together by electri- 

 city, the oxygen being negative and the hydrogen positive, 

 to such a degree that they exactly balance each other, and 

 produce no free electricity. This idea is shown in Fig. 117 

 with five neighbouring atoms of water, of very exaggerated 

 dimensions of course. 



The electro-motor is partly plunged into the water. It 

 becomes polarised ; the part immersed is positive, and 



