198 Dr. Matthiessen on Standards of Electrical Resistance, 



One glance at the foregoing Table will suffice to show how 

 very badly Lenz's series agrees with the rest when mercury is 

 taken as unit ; and comparing Becquerel's and my own, our 

 values differ for — 



Table IV. 

 When silver = 100. When mercury = 100. 

 Copper .... 4*3 per cent. 15 per cent. 



Gold 11-6 „ 21-6 „ 



Cadmium . . . 9*0 „ 2 - „ 



Zinc 11-9 „ 22-1 



Tin 180 „ 8-0 „ 



Iron 9-0 „ 192 



Lead 5*7 „ 5*9 „ 



Platinum. . . . 18-1 „ 27-3 



These results prove that the mercury standard proposed by 

 M. Siemens cannot be a useful and good one ; for, in fact, we 

 obtain more concordant results if we take in the above series any 

 other metal as unit. The mercury employed by three of the 

 observers was stated by them to have been pure. 



3. Page 93 (2nd paper), M. Siemens states, "German-silver wire 

 is much better for resistance coils than the gold-silver alloy, on 

 account of its high conducting power and expense." I quite agree 

 with him. 1 only proposed the gold-silver alloy to be used for 

 the same purpose as he does mercury. 



4. Page 93 (2nd paper), M. Siemens states, " Even if the con- 

 ducting power of the gold-silver alloys were the same, yet small 

 resistances cannot be accurately compared with them, as there 

 would always be a slight difference in the resistance at points 

 where the alloy is connected with the connectors of the apparatus" 

 I may, however, mention that I always solder the ends of the 

 normal wire to two thick copper wires (of 2-3 millims. diameter 

 and about 35 millims. long), the free ends of which are carefully 

 amalgamated by dipping them into a solution of nitrate of mer- 

 cury in dilute nitric acid ; and the connexions are made by means 

 of mercury cups, the bottoms of which are amalgamated copper 

 plates. These can be removed, and are of course from time to 

 time reamalgamated. The free copper ends of the normal wire 

 are reamalgamated every time before use. This arrangement gives 

 most satisfactory results ; not the slightest change in the resist- 

 ance is observed when the normal wire is taken out of the mer- 

 cury cups and put in again. If, however, a wire of the gold- 

 silver alloy has once been made and arranged for use, when 

 wanted it is only necessary to reamalgamate the ends, and it may 

 then be used without further loss of time. On the contrary, for 

 M. Siemens' s proposed unit there must be a great deal of time 



