44 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
This was found to be so exactly the case that though H was provided with a vernier 
reading to yg-tli of a millimetre, and the addition of the yqth ohm required H to be 
shifted through about 300 millims., no difference whatever could be detected between 
n i ~ n 3 and n 2 —n 1} the galvanometer being sufficiently delicate to show an alteration of 
resistance equal to that of y^th of a millimetre of N N. In the same manner each 
portion of the wire was tested, and it was concluded that no difference of resistance 
amounting to 1 in 3000 existed in any part of the graduated wire. 
The uniformity of N N having been established, it was easy to compare the tenths 
with each other, and by determining the exact value of k, to find the values of their 
resistances in terms of the divisions of the platinum-iridium wire : this was accordingly 
done, and the values thus obtained were ascertained to accord fairly well with each 
other. 
In order to secure still greater accuracy, the wire N N and the whole of the 
resistance-coils were afterwards tested by means of a second box of resistance-coils,'" 
some eight or nine entire days having been spent in this work. These fresh trials 
confirmed the results obtained in the previous ones as regards the uniformity of N N 
and the relative values of the tenths; but the absolute values of the latter in terms of 
the divisions of N N were found to vary slightly on different days, as also did those of 
the rest of the resistance-coils. Thus the sum of the ten resistances in the units dial- 
plate were found on three separate days to be equal to that of 58,605, 58,739, and 
58,437 divisions of N h respectively.! 
From these and other experiments the average value of the resistances of each of 
the units at 17° C, at which temperature a unit agreed, according to Kieser, with 
1 ohm, was equal to the resistance of 5859 - 4 millim. divisions of the platinum- 
iridium wire. 
The units accorded very well with each other and with all the other resistances in 
the box except the tenths, nearly all the latter being slightly too low. The exact 
values, however, of each of the units and tenths were tabulated and used in calculating 
the results of the different experiments made in this and subsequent parts of the 
enquiry. 
* This box was kindly lent to me by Mr. Kieser, of Elliott Bros., to whom I am also indebted for 
diagrams 7 and 8. 
t These variations are certainly not due to errors of observation, as the results of trials made within 
one or two hours of each other agreed much more closely with each other; they may be attributed almost 
entirely to the prevalent plan of embedding resistance-coils in solid paraffin, whereby the temperature of 
the coils inside the box is frequently very different from that of the air outside : this plan is, I am con¬ 
vinced, a very bad one when great accuracy is required, as not only do the wires get heated to an extent 
which is very appreciable, even when only a single cell is employed, but a “Peltier effect” is produced at 
the junctions of the coils and the brass-blocks which can never be properly got rid of; and thus the labour 
and care bestowed on these resistances are to a great extent lost. It would be better to fill the box with 
some such liquid as paraffin oil. 
