Hf, A. Rowland—Absolute Unit of Electrical Resistance, 327 
which does not contain H or H’, and the distance of the mirror 
from the scale does not enter except as a correction in the ratio 
of sin $6’ and tan a’; and, as a and 6 can be made nearly 
equal, the correction of the tangent galvanometer for the 
length of needle is almost eliminated. When the method of 
1 
recoil is used, we must substitute 1+4 A\° for the term in- 
7 
volving A, and sin $A’+sin $B’ in the place of sin 46, A’ and 
B’ being the greater and smaller arcs in that method. This is 
on the supposition that A is small. 
The ratio of G’” to G must be so large, say 12,000, that it is 
difficult to determine it by direct experiment, but it is found 
readily by measurement or indirect comparison. — 
It is seen that in this equation the quantities only enter as 
the first powers, and that the only constants to be determined 
which enter the equation are M, G and @”, which all vary in 
simple proportion to the linear measurement. It is to 
noted also that the only quantities which require to be reduced 
to standard measure are M and T, and that the others may all 
be made on any arbitrary scale. No correction is needed for 
temperature except to M. Indeed, I believe that this method 
exceeds all others in simplicity and probable accuracy and its 
freedom from constant errors, seeing that every quantity was 
varied except G’” and G, whose ratio was determined within 
probably one in three thousand by two methods. 
Having obtained the resistance of the circuit by this method, 
we have next to measure it in ohms. For this purpose the 
resistance of the circuit was always adjusted until 1t was equal 
to a certain German silver standard, which was afterward care- 
fully compared with the ohm. This standard was about thirty- 
five ohms. 
By this method, the following data are needed. 
1. Ratio of constants of galvanometer and circle. 
2. Ratio of the tangents of the two deflections of tangent 
galvanometer. : 
8. Ratio of the deflection to the swing of the other gal- 
vanometer, : 
4. Mutual potential of induction coils on each other. 
5. Time of vibration of the n 
6. Resistance of standard in ohms. 
For correction we need the following: 
1. The logarithmi 
4. Rate of chronometer. 
5. Correction to reduce to standard meter. . 
