174 MR. A. NORMAN SHAW: A DETERMINATION OF THE ELECTROMOTIVE 
standard at the certified marks gave, at different temperatures, the following results 
which are reduced in each case to their value at 19°'0 C. :— 
Table X. 
Absolute length of 100 cm. on standard brass scale at 19° - 0 C. 
cm. 
100-0297 
100-0293 
100-0297 
100-0290 
100-0286 
100-0294 
100-0293 
Mean. 100-0293 
It is necessary to test the assumption that the edges of the cylinders as seen in the 
comparator microscope lay quite in the plane of their ends. Comparisons were made 
by placing them end to end and pressing together tightly, then (l) measuring the 
distance between marks made on each cylinder, at this position; (2) placing the 
cylinders at the extreme position and measuring the distance between the marks, and 
(3) measuring the distance between the inner edges at the external position. The 
difference of the first two readings should equal the latter measurement if the sighted 
edge is quite at the extremity of the cylinder. Measurements of this kind were made 
with the weights in various rotated positions in the tube and a very good agreement 
was found. The variations between the results of the two methods were only of the 
same order as those obtained for different determinations of the distance between the 
marks at the inside position when the cylinders were placed end to end. The 
variation in this case was caused mainly by the difficulty of keeping them quite 
together without maintaining pressure. As the method with marks involves two 
measurements it will be seen from the figures below that it is more satisfactory to 
take the observations by sighting on the edges of the cylinders. The best and worst 
agreements of the two methods which were obtained were:— 
Table XI. 
Distance by method with 
marks. 
Distance by sighting on 
edges. 
cm. 
cm. 
(1) 30-2878 
30-2877 
(2) 30-2889 
30-2878 
Great care was taken to maintain constancy of temperature during all the linear 
measurements and in every case the lengths are reduced to their value at 20"0 C. in 
standard centimetres. 
