140 EEPOET— 1892. 



stone's bridge arrangement. As regards the variation of resistance 

 through winding, it was observed that the resistance of all kinds of wire 

 increased by winding, as would be expected, the increase being more pro- 

 nounced for a given gauge of wire the less the bobbin's diameter. This 

 increase is due to a mechanical hardening of the wire by strain, and it is 

 well known that the resistance of any metal is less in the annealed state 

 than in the hardened condition. At the same time an elongation can 

 hardly be avoided, especially with thin wires, also causing an increase 

 of resistance. As the gauge of the wires generally was 1 mm., this 

 second cause was of secondary importance. 



In the first place we investigated a German-silver alloy which the 

 firm of Siemens & Halske in Berlin used for standards at that time. It 

 appeared that the increase of resistance through winding could amount 

 to 1 per cent., and that the time-variation during the following months 

 was very considerable ; the latter showed itself always as an increase of 

 resistance. Another remarkable circumstance is the further increase of 

 resistance (amounting to a few tenths per cent.) by heating such a wire 

 for several hours at about 100° C. 



It might be supposed that the wire would be annealed by the effect of 

 the high temperature, and that its resistance would therefore decrease. 

 But our extensive investigations gave the important i^esult that heating 

 causes an increase of resistance in all alloys containing zinc to any con- 

 siderable amount. On the other hand, all alloys examined containing no 

 zinc show a decrease of resistance under the same conditions. The 

 increase of resistance by winding is also much more pronounced with 

 alloys containing zinc than with those in which this metal does not occur. 

 All this seems to point out that in the former alloys changes of structure 

 go on, which are accelerated by any kind of stress or by variations of tem- 

 perature, and always tend to increase the resistance. These changes of 

 structure also become apparent by the time- variations, which occur when 

 tlie resistance coil is left to itself. The latter observations are in perfect 

 agreement with what was found by former observers on the time- variation 

 of German-silver. The interesting result was then arrived at, that the 

 time- variation would be much accelerated by heating the resistance at a 

 high temperature, say 100° C, for a few hours. Within two months 

 after winding, the period in which German-silver varies most, variations 

 •could not be shown within the errors of observation in wires treated in 

 the manner described. During longer periods, say one or two years, 

 variations would still occur, even with annealed German-silver coils. But 

 they hardly reach the tenth part of those occurring when this process has 

 not been gone through. 



The following table shows the results of one of the experiments with 

 two wires of German-silver (60 per cent. Cu ; 25 '4 per cent. Zn ; 14'6 

 per cent. Ni), the specific resistance of this material being 30 microhms 



per 2? a-nd its temperature-coefficient 0'036 per cent, per degree Cen- 

 tigrade. In both cases the wire (1 mm. in thickness) was wound on 

 a bobbin of 10 mm. diameter. The wire marked I, was left to itself 

 after winding, whereas the wire II. was annealed after winding by 

 heating it to a temperature of 90° Centigrade during three hours. The 

 resistance of each was measured at intervals of nearly two months 

 from time to time. 



