( 678 ) 
can be used at a distance of 10 cms. In this case one revolu- 
tion of the head (divided into 100 parts) of the micrometer screw 
(ef. Comm. No. 60 § 15) was equivalent to 60 to 70 u. The levels 
on the telescopes were carefully calibrated; at the distance used, one 
division on the levels corresponded to from 4 to 6 w and the uncer- 
tainty in reading was less than 0.2 division or about 1 u. After each 
setting, 80 seconds was allowed to elapse before reading and former 
measurements have shown that this is sufficient for the attainment of 
equilibrium. 
The field of view of the microscopes was also investigated by 
measuring at various points a '/, mm. scale, but no irregularity could 
be found. 
4th, Measurement of resistance. The doubled conducting wires 
a, 6, c ete. at the ends of each measuring wire AB, etc. (cf. Pl. I) 
were lead to eight cups of mereury for each rod, which cups could 
be connected in pairs to the wires from the Wueartsronr bridge. 
sy measuring 
w,=a+AB+b 
w,=—ce+ AB4+d 
wadde 
I= bd 
the resistance of the wire AB 
w,+w,—w,—w, 
Win - 
9 
i 
can be determined *). The galvanometer with reading scale (see 
Comm. N° 25, April ’96) had a resistance of 6 w and a sensitiveness 
of 2.5 > 107. Thermoelectric forces in the circuit of copper leads, 
platinum leads and platinum resistances are unavoidable, they were, 
however, onty small and could be eliminated. 
§ 8. Survey of a determination. A complete determination com- 
prises focussing the microscopes, referring to the standard scale, 
and reading the thermometers, as well as the various determinations 
of resistance between A and Bb, B and C, C and D. 
In the following table all the readings for the determination of length 
of the Jena rod in liquid oxygen are given. Column A contains the 
readings of the micrometer heads, 6 the corresponding positions of 
the levels, C the nearest division on the standard scale, D and H# 
the micrometer and level readings for this and / the temperatures. 
1) In our case the influence of the shunt between A and B, C and D was so 
small that it could be neglected and then ws + w4 could be determined at once. 
