NATURAL PHILOSOPHY. 119 



is slow by comparison with that which ensues when, owing to a flaw 

 or disintegration of the covering tin envelope, atmospheric agencies 

 come into play upon the underlying metal. 



But the case we most particularly wish to direct attention to, is the 

 result of bringing zinc into contact with iron, and retaining the two 

 metals together over long periods, as may be seen in the so-called gal- 

 vanized iron. Under this latter disposition zinc becomes the sacrifi- 

 cial metal. Not one particle of iron decays so long as atmospheric 

 destructive agents can wreak their dissolution so to speak on the 

 protective zinc. Nor does this protection altogether depend on a 

 complete covering of the iron. Flaws and imperfections there are, 

 and necessarily must be, through which, quite down to the iron, de- 

 structive agents, always present in the atmosphere, must penetrate. 

 They do not act upon the iron, nevertheless, so long as a sufficient 

 expanse of protective zinc surface remains ; and this simply because 

 a fundamental law of nature forbids their doing so. London Me- 

 chanics 9 Magazine. 



NEW APPLICATION OF ELECTEICITY. 



Mr. Siemans, the well-known English electrician and physicist, in 

 a late communication to the Philosophical Magazine, thus describes 

 a striking and most fortunate application of a known fact in electri- 

 city. We usually figure this agent as a fluid, and somewhat grossly 

 imagine it to pass through conductors as water does through pipes. 

 The symbol does no harm so long as we know that it is a symbol ; and 

 the friction of a liquid in passing through a narrow pipe, answers, 

 perhaps, sufficiently well to illustrate the resistance encountered by 

 an electric current in passing through a wire. The longer the wire 

 is, the greater is the resistance ; and wires of the same thickness and 

 length, but of different chemical constitution, offer different amounts 

 of resistance to the passage of an electric current. A wire of plati- 

 num, for example, one yard in length, will throw the same obstacle 

 in the way of a current as a wire of silver twelve yards in length and 

 of the same thickness. But besides difference of dimension or differ- 

 ence of chemical quality, there is another circumstance which influ- 

 ences the conductive power of a wire, and that is its temperature. 

 As a general rule, when the temperature of a wire augments, its 

 resistance to the passage of an electric current augments, or, in other 

 words, its conductivity becomes diminished. We can express the 

 resistance of any wire in numbers referred to a fixed standard. We 

 can also determine with the utmost exactness the augmentation of 



^j 



electric resistance due to any given augmentation of temperature. 

 Conversely, we can accurately infer the increase of temperature from 

 the increase of resistance ; and this is the principle which Mr. Siemans 

 has so happily applied. He had charge of the Rangoon and Singa- 

 pore telegraph cable, and was led by previous observation to surmise 

 that a spontaneous generation of heat sometimes took place when 

 large lengths of such cables are formed into coils. He was, therefore, 

 anxious to keep himself acquainted with the temperature of the inte- 

 rior portions of his coil, but could not, of course, introduce ordinary 

 thermometers there. He introduced, however, between the layers 



