4 Dr. F. Auerbach on the Passage of 



I commence with the examination of the influence of mag- 

 netization upon the resistance. 



For measuring the resistance I used a Wheatstone bridge. 

 A universal resistance-box by Siemens furnished the resist- 

 ances w^ ic 2 , io 3 of the three parallel branches. In the expe- 

 riments the ratio u\ : ic 2 was almost constantly as 1000 : 10, 

 in some instances as 100 : 10 ; consequently the resistance 

 ic 3 in the first case represents 100-fold the resistance w 4 to be 

 measured. Since 0*01 of a Siemens unit can be measured with 

 certainty, the accuracy of the determination extends to the 

 fourth decimal place ; it only became uncertain in case w 4 

 amounted to more than 10 units. 



I at first used as the measuring-instrument a Wiedemann 

 galvanometer with the thinnest of the wire coils that are usu- 

 ally joined to it, afterwards one constructed by Sauerwald 

 according to Magnus' plan. By means of mirror-reading I 

 could estimate with certainty 0*1 of a scale-division ( = 0*1 

 millim.). The last-mentioned instrument is highly suitable 

 for investigations in which rapidity of observation is import- 

 ant, on account of the quick suppression of the vibrations. 



The first and most difficult problem was, hoiv to exclude the 

 influence of variations of temperature ; for when we consider 

 that according to the statements of Thomson, Beetz, and 

 Chwolson the upper limit of the alterations of resistance by 

 magnetizing is given by the ratio 1 : 1000, and that an alte- 

 ration of resistance corresponding to this limit value is pro- 

 duced in iron by a change of temperature of o, 2 C, it is evi- 

 dent what careful attention ought to be directed to this point. 

 The effects of temperature can either be set aside by a suitable 

 arrangement of the experiment before the commencement of 

 the observations, or eliminated by calculation after their con- 

 clusion. In regard to that portion of those influences which 

 is produced by the principal current itself, after some prelimi- 

 nary experiments I decided for the latter. Of course, in the 

 equation of the Wheatstone bridge, I could easily have made 

 the ratio n\ ; w z independent of the temperature by having 

 the resistance w 3 for the most part of iron, and taking only the 

 small part requisite for the actual regulation out of the resist- 

 ance-box. But since the ratio w 2 : w 1 was = 1 : 100, iron 

 wires of considerable length would have been necessary, which 

 for other reasons I was obliged to avoid. Moreover the heat- 

 ing produced by from 1 to 3 Daniell elements, such as I used 

 for the principal current, during the mostly very short time 

 that the current was closed, was extremely little ; and it is 

 well known that with small values of the foreign influences (of 



