Strength of field. 

 M. 



Magnetism on a permanent Electric Current. 315 



Replacing now the old perforated poles of the electro- 

 magnet by solid new ones, and removing one or two other 

 sources of error, I found: — 



March 5th, ^^ = 150 x 10 10 



— 1 54 

 Mean . =1513x10° 



The strength of the magnetic field was, as usual, determined 

 twice on March 5th, once before and once after the other ob- 

 servations. The two values varied by something more than 

 1 per cent. The mean of the two is taken as the uniform 

 strength for the day. The strength of the primary current 

 sent through the gold strip was much varied for the different 

 series of observations. 



Thus we may write as corresponding to the above three 

 values: — 



Strength of primary 

 current. 



c „ , 



6400 *xtan23 44 



„ „ 42 14 

 „ „ 49 28 



when k is the constant of the tangent galvanometer = *07 

 nearly. 



The agreement between the mean of the various results 

 previously obtained and the mean of those found March 5th 

 was considered satisfactory; and the next measurements were 

 made with 



Gold Foil, No. 5. 



The metal in this plate was, I believe, either " soft " or 

 " semicohesive." 



Length of strip when weighed = 8 # 49 centims. 



Width „ ,, = about 3*28 „ 



Area including projections = 30*0 square centims. 



Weight = -1122 grm. 



Thickness = '000188 centim. 



This strip, after being placed on the glass, was trimmed 

 down to a width of about 2'32 centims.; and the mean thick- 

 ness of this strip was no doubt quite different from the value 

 above obtained. This strip was reduced in width, after being 

 weighed, more than any other that has been used ; and this 

 fact may account for the discrepancy between the results 



