332 H. A. Rowland—Absolute Unit of Electrical Resistance. 
e. Correction of T for the are of vibration. This are was always 
the same, starting atc, and being reduced by damping to aboute,, 
—_— Tue = 
c= + iyenn Ca” —%")s 
where c, and ¢, are the total ares of oscillation. 
f. Correction for length of needles. For the tangent gal- 
vanometer, the correction is variable. For the circle it is 
q 
S= +1( : 
where 7 is half the distance between the poles of the needle and 
A radius of circle. For the other galyanometer it 1s 
included in the formula for G. 
uction to normal meter. As the dimension of R is a 
velocity and the induction coils were wound on brass, the cor- 
rection is 
=-+-y('—?’) 
where y is the coefficient of expansion of brass or copper, ¢ 
the actual and ¢” the normal temperature. 
orrection of standard resistance for temperature. Let 
M be the variation of the resistance for 1° C., ¢’” be the actual 
and ¢v the normal temperature 17°°0 C.; then 
B>=— p(t" —#"), 
C. Correction for length of needle in tangent galvanometer, 
x 15. ' e\ ! 
Vas += sin (a+ a)(r) (a’—a) 
where 7’ is half the distance between the poles of the needle 
No. 3. By the introduction of commutators at various points 
all ge isturbance of instruments could be compensated. 
io. a . . 
No. 6. The circle was always adjusted parallel to the coils of 
the galvanometer. Should ‘hes not be parallel to the needle, 
G and G” will be altered in exactly the same ratios and will thus 
not affect the result. The same may be said of the deflection 
of the magnet from the magnetic meridian due to torsion. 
