984 H. A, Rowland—Absolute Unit of Electrical Resistance. 
sultant action is such that the lines of force are apparently 
dragged after the body as though they met with resistance 
in passing through it: and so we may regard Thomson's 
method as a means of measuring the amount of this dragging 
action. 
' But, however beautiful and apparently simple the method 
may appear in theory, yet when we come to the details we find 
many reasons for not expecting the finest results from it. 
Nearly all these reasons have been stated by Kohlrausch, and I 
ean do barely more in this direction than review his objections, 
point out the direction in which each would affect the result, 
and perhaps in some cases estimate the amount. 
In the first place, as the needle also induced currents in the 
coil which tended in turn to deflect the needle, the needle must 
have a very small magnetic moment in order that this term may 
be small enough to be treated as a correction. For this reason 
the magnetic needle was a small steel sphere 8 mm. diameter, 
and not magnetized to saturation. It is evident that in a qui- 
escent magnetic field such a magnet would give the direction 
of the lines of force as accurately as the large magnets of Gauss 
and Weber, weighing many pounds. But the magnetic force 
due to the revolving coil is intermittent and the needle must 
show as it were the average force, together with the action due 
to induced magnetization. Whether the magnet shows the 
dragged with the coil, and hence makes the deflection greater 
than it should be, and the absolute value of the Ohm too small 
e. 
: The mere fact that this small magnet was attached to a com- 
paratively large mirror which was exposed to air currents 
could hardly have affected the result, seeing that the disturb- 
ances would have been all eliminated except those due to air 
currents from the revolving coil, and which we are assured did 
not exist from the fact that no deflection took place when the coil 
was revolved with the circuit broken. In revolving the coil in 
posite directions very different results were obtained, and the 
