208 



ON MAGNETISM. 



Fig. 68. 



ceptibility to the action of the acid : great care is taken 

 to prevent the plates from touching. For one metal, 

 zinc (its surface usually being rubbed with quicksilver) 

 has. from the beginning of the science, been adopted : 

 for the other metal, silver, or copper, or (now more 

 commonly) graphite, a form of carbon extracted from 

 the iron retorts in which coal is distilled in the manu- 

 facture of gas for illumination. These two plates are 

 connected by a metallic wire: or, a separate wire is 

 soldered to each plate and the wires are brought into 

 mechanical contact (which, if the touching surfaces are 

 clean, is sufficient). Then a galvanic current or gal- 

 vanic currents pass through the wire. We are justified 

 in asserting this by observing that heat is produced in 

 the wire, sometimes sufficient to fuse iron and platinum : 

 that magnetic effects (to be mentioned shortly) are pro- 

 duced at every part of the wire: that time is required 

 for the transmission of the effect through great length 

 of wires : and that the disruption of the wire at any 

 point produces a spark. The phenomena seem to 

 justify us in asserting that a current proceeds from 



