112 



HENRY A. KOWLAND 



column is the sum of two results with the current in^ opposite direc- 

 tions, and after letting the bar stand for some time, as indeed was done 

 in nearly every case. The first column contains twice the quantities 

 observed, so as to compare with the others. The zero-point was at the 

 end of the bar covered by the helix. 



The value of Q"^ between and 1 includes the lines of force passing 

 out at the end of the bar, and is therefore too large. 



In Fig. 4 we have a plot of the results found for this bar. The 

 curves are such as we should expect from our theory, except for the 

 variations introduced by the causes which we have hitherto considered. 

 Thus the sharp rise in the curve when near the end of the bar has 

 already been explained in connection with Table III. A small portion 



FIG. 4. Plot of Table X. 



of it, however, is due to those lines of induction which pass out through 

 the end section of the bar; and in future experiments these should be 

 estimated and allowed for.* 



To estimate the shape of the curve theoretically in this case, let us 

 take equation (4) once more, and in it make s'=oo and s" = \/TZR', 

 which will make it apply to this case. We shall then have A' = 1, 

 and A" =o>, whence for the positive part of Q' f ' we have 



2R'r l 

 and for the negative part 



(1 + e* 



_ -rxN . 



8 When considering surface-density, we should also allow for the direct action of 

 the helix, though this is always found too small to be worth taking into account 

 except in very accurate experiments. 



