H. A, Rowland—Magnetic Proof Plane. 15 
surface density of the magnetism at that point will be nearly 
proportional to the deflection. In the case of an electro-magnet 
the surface density will be nearly proportional to the deflection 
minus the deflection which would be produced by the helix 
alone, though the last is generally pte et may be neglected. 
use the words nearly in the above statement because they are 
only exactly true in the cases where the lines of force proceed 
from the surface in a perpendicular direction; otherwise the 
deflections must be multiplied by the secant of the angle made 
by the lines of force with the surface of the magnet. In the 
Having obtained the distribution for any given magnet, the 
distribution for any similar magnet of the same material but of 
different size becomes known by a well known law of Sir 
William Thomson. 
As, in the present state of our knowledge, magnetic measure- 
ments are of small value unless made on the absolute scale, we 
require to reduce our results to this system. There are seve 
methods of doing this, but the simplest is that which I have 
used in my experiments on magnetic permeability, and consists 
™m including an earth inductor in the circuit. A coil laid on a 
perfectly level surface is sufficient for this: when this is turned 
over, the induced current will be equal to C= st 
is the number of turns in the coil, A its mean area, V the ver- 
tical component of the earth’s magnetism, and R the resistance 
of the circuit. When the small coil is pulled suddenly away 
the current will be C’=™ aa and so we have Q=2V— 
which when a Thomson galvanometer is used C’ and C can be 
R an'C' 
replaced by the corresponding deflections; hence Q=2V—;— 
, where n 
in 
* On a new diamagnetic attachment to the lantern, &c., this Journal, May, 1875. 
