ON STANDARDS OF ELECTRICAL RESISTANCE. 365 



Besidts. 



Eesistance of tube No. I. 1-00000 

 „ „ No. II. 0-99849 



» }> 



No. III. 1-00000 



An approximate table is subjoined of the resistances of a metre-gramme 

 of the different metals in terms of the B. A. unit, 1864. 



Copper 0-1469 



Silver 0-1682 



Gold 0-4150 



Gold-silver alloy 1-668 



Lead 2-257 



Mcrcmy , 13-06 



From the foregoing results we may draw these conclusions. 



That with great care an unit may be reproduced with great accuracy by 

 anj' of the metals or alloys above mentioned. 



Of those tested it appears that lead is the most preferable on account of 

 its easy purification, and because the presence of impurities, amounting to 

 several per cent., produce no very disproportionate effect on its conducting- 

 power. For instance — 



Conducting-power of lead is 7-77 



Of lead with 12-7 per cent, volume of tin is 8-13 



„ 10.6 „ „ cadmium .... 8-38 



„ 2-3 „ „ bismuth 7-0 



„ 3-8 „ „ antimony .... 7-1 



„ 2-3 „ „ silver 7-9 



"With the other metals and alloys tested a much greater difference is found 

 in the conducting-power when such impurity exists. 

 A few examples wiU show this. 



The conducting-power of pure copper is 100 



Of copper with 1-6 per cent, in volume of silver 65 



Of silver with 1-2 „ „ gold 59 



Pure silver being taken as 100. 



Of gold with 1-2 per cent, of silver 73-8 



The conducting-power of pure gold being 78. 



If the conducting-power of mercury is 10-9 



That of mercui-y with 1-18 per cent, volume of lead is . . 11-5 



„ 1-8 „ „ tin 11-8 



„ 1-8 „ „ zinc .... 12-4 



0-7 „ „ gold.... 11-6 



„ 1-2 „ „ silver .. 11-6 



The manipulation with lead is rendered easier by its high resistance. 



Mercury is easily purified, and good results are always obtained with 

 it. It would, however, in reproducing an unit, be necessary to distill the 

 mercuiy, because traces of such impurities as silver and gold, which may 

 easily get into it when in use in a laboratory, cannot be removed by treat- 



