SCIENCE AND PRACTICE. 275 



with, the view of determining the same question, completely 

 corroborate. We connected a finely -adjusted annealed 

 German-silver (wire) resistance to a self-acting make-and- 

 break apparatus, or " Wippe," which sent reversed currents 

 from a battery of large DanielFs elements, at an immense 

 speed through it both night and day. In addition to this, 

 the wire was kept in a recess in an iron stove in the 

 laboratory, so that, without any interference on our part, 

 its temperature was raised by day at least to the tempera- 

 ture of boiling water, and during the night descended to 

 within a few degrees of the freezing point. The battery 

 was varied at intervals from one cell to twenty, during 

 about six weeks, but the conducting power of the metal 

 did not vary in the least. Suspecting that this constancy 

 might be due to the reversals, we repeated the experi- 

 ment with a zinc current made and interrupted with great 

 rapidity, with the same result. 



The belief in the inconstancy of metals may have its 

 origin in the discovery that all the old resistance- scales 

 and rheostats are no longer exact. This may have its 

 origin in three causes : 1st, the greater perfection of our 

 systems of measurement enabling us to detect differences 

 which may have existed before, but which we were unable 

 to appreciate ; 2nd, the process of annealing, which, when 

 commencing with a hard- drawn wire, may extend over a 

 very long time if the wire is only exposed to variations of 

 the temperature of the atmosphere ; and 3rd, the oxidation 

 of the surface when the air has access to it. 



There can be no question that much has still to be 

 learned in this branch of science. Wires of German-silver 

 and some other alloys become, when exposed freely to 

 the air for some years, so brittle as to be incapable of being 

 wound up on reels without danger of occasional ruptures 

 of continuity. That this brittleness is accompanied by a 

 change in conducting power is probable. It remains, how- 

 ever, to find whether the exclusion of air by means of some 

 such material as paraffine will prevent the brittleness in ques- 

 tion, or if it is due to molecular changes of the alloy. 



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