METALS FROM DESTRUCTION. 101 



the copper from sea-water, it will be found that to obtain the same 

 amount of electricity by iron and zinc, 2 square feet of the latter to 

 4 square feet of the former are required. 1 Besides which, the same 

 quantity of electricity that protects copper will not protect iron; nor 

 will any quantity of zinc protect iron from corrosion in sea-water 

 even a bar of iron placed in a zinc vessel filled with sea-water is not 

 completely protected. 



2dly. The conducting power of the negative or protected metal 

 subjected to submarine immersion is a subject of very great import- 

 ance. Suppose a piece of copper and a piece of zinc be connected 

 under a solution say a copper bar (c) 4 feet long, with a piece of 

 zinc (z) 4 inches in 

 length, erected on 

 one end, as in the 

 annexed sketch : 



The conducting 



power of the copper c 



is so much superior 39. 



to that of the solution 



that the whole length of the bar will become instantly negative, 

 and the current of electricity will pass to and from all parts of the 

 bar at the same time in the lines , 5, b ; but the current will be 

 more active towards the point of contact than towards the distant 

 extremity the resistance of the solution being less in proportion 

 to the proximity of the metals. But if a bar of iron, and a piece 

 of zinc as a protector, be placed in the same circumstances, the 

 phenomena assume quite a different aspect : the conducting power 

 of iron being much less than that of copper, the distant extremity 

 will not be affected by the electric current, which will find a more 

 easy passage, as indicated by the dotted lines e, e, e, beyond which 

 the electric effort ceases ; and even in that portion of the bar which 

 is under the influence of the current, the part nearest the zinc is 

 better defended than those parts which are farther distant. This 

 partial protection, while it induces a negative state at the near end, 

 renders the other end more positive. Such a diversity of condition 

 gives rise to voltaic action between the two extremities of the bar, 

 and the result is the destruction of the far end. In all cases of vol- 

 taic protection the more equal the influence over the whole surface 

 protected the more perfect is the protection. An inequality of pro- 

 tection, such as we have described, is productive of numerous evils. 

 It is, we believe, the source of many of the injuries occurring in our 

 day to copper sheathing. One part of a sheet becoming, by some 



1 These proportions, given in round numbers, are nearly accurate; but they 

 vary according to the kind of iron, the state of the water, the distance of the 

 metals, &c., &c. 



