146 



REPORT 1892. 



the electromotive force of the standard Clark cells with the silver- 

 voltameter. Thus very often (more than fifty times) a current of 

 about one half of an ampere was passed through it for one hour each 

 time. At 18° C. I found the following values : — 



Tadle VII. 



Again, in the following table are stated in microhms the differences 

 in the resistance of four manganin standards (No. 148 to No. 151) of 

 one ohm. The numbers marked * were observed by Drs. Kreichgauer 

 and Jager, using Kohlrausch's differential galvanometer method, the 

 others by myself, using a Wheatstone's bridge arrangement. 



]V] easurements were also made of these standards shortly after their 

 construction in July 1891, but not with quite the same accuracy as the 

 later ones. Anyhow, they show, in connection with numerous com- 

 paripons of the four coils with other standards, which were checked by 

 mercury resistances, that the manganin coils were constant for the space 

 of ore year within a few thousandths per cent. 



The patterns referred to are intended to be standards oi resistance. 

 On the other hand, resistances of 0-01, O'OOl, and even 0001 ohm are 

 used for measuring large currents up to a few thousand amperes by 

 corof ensating the potential difierence which the current itself produces 

 in flowing through the resistance. These resistances consist of manganin 

 plates, which are soldered with silver to stout copper bars. The dimen- 

 sions of the plates are chosen in such a manner that the value of the 

 resistance is too small to begin with, and the definitive adjustment is 

 arrived at by boring small holes in the plates ; the latter are again 

 coatf d with varnish in order to protect thpm against any chemical action 

 of the oil, and so on. For uncovered wires, as they are used, for instance, 

 in 1 ridges, or in technical resistance?, the manganin is perhaps not so 

 appropriate as the alloys commonly used. For all other resistances, 

 b<'W< ver, we think it is the best alloy hitherto known, because it facili- 

 tati s the electrical measurements, and brings them to a higher degree of 

 accuiacy than was formerly attainable. 



