Equipotential Lines of cm Electric Current. 135 



Norway iron. — Eight crosses of iron were used in such 

 tests. Nos. 1, 3, 5 and 7 are broad ; Nos. 2, 4, 6 and 8 are nar- 

 row. Eos. 1-4 were cut frohi one bar of ordinary [Norway 

 iron such as blacksmiths use. Nos. 5-3 were cut from another 

 bar of the same kind. 



Of these bars the ends, which had possibly been heated and 

 hammered more than the other portions, were avoided in cut- 

 ting out the crosses. The dimensions of the crosses, as given 

 below are only roughly correct. 



No. 



Width. 





Thickness. 



Le] 



igth. 



Main part. 





Arras. 



Main part. 



Alain part. 



Arms. 



[near arms.] 



[near 



main part] 



[near arms.] 





[each.] 



1 7'0 mra 





2'0 mm 



2'20 mm 



4Qmm 



gmm 



2 0-5 





0'5 



2-30 



42 



9 



3 9'5 





2-0 



2-86 



43 



4 



4 0-5 





0-5 



2*85 



42 



7 



5 8-0 





2-0 



0-50 



3J 



4 



6 0-5 





0*5 



0-48 



30 



9 



7 9'0 





2-0 



3*00 



38 



5 



8 0-5 





0-5 



3-00 



33 



9 



Nos. 1 and 2 were compared June 18th and 19th, 1885. 

 The strength of magnetic held used is not recorded. The 

 comparison, which was very rough, gave 



R. P. in narrow cross . 



R. P. in broad cross 



Nos. 3 and 4 were compared in August, 1885, in a magnetic 

 field the intensity of which was about 8000 or 9000 (c. g. s.). 

 This test, which also was rough, gave 



R. P. in narrow cross 

 — 2»g 



R. P. in broad cross 



Nos. 5-8 were tested Dec. 31st, 1885, and Jan. 1st, 1886. 

 The order of trial on Dec. 31st was Nos. 7, 8, 6, 5, 5, 6, 8, 7. 

 The purpose of this arrangement was to eliminate, so far as 

 practicable, from the result of the trial error arising from 

 changes in certain factors of the test during the progress of 

 the experiment, such as a possible variation in the sensitiveness 

 of: the astatic galvanometer used to measure the transverse cur- 

 rent or a gradual weakening of the magnetic held. An occa- 

 sional observation of the time of vibration of the needle in the 

 astatic galvanometer showed that no great change was occur- 

 ring in the sensitiveness of this instrument. The weakening 

 of the magnetic field in the course of the day was probably 

 less than 10 per cent. By combining the two observations 

 made with each cross four results are obtained which should 



