410 
MR. E. H. GRIFFITHS ON THE YALUE OF 
As the terminals were used for testing at the time the box was constructed, 
the resistance of the connections through the dials was then taken. This is sufficient 
to account for the apparently low value of the first unit, as the resistance of the box 
with all the plugs at 0 is ‘OOSST. In testing the arms of the bridge, the centre and 
end terminals were used in the same manner as when the box itself formed the bridge, 
so that no plug resistance correction had to be applied. 
A table was constructed, giving the total difference between the reading and the 
real value (in terms of legal ohms) for every position of the plugs in each dial. This 
difference we termed the “ plug correction.”* Having made this correction, we then 
had to correct for any inaccuracy in the ratio of the arms of the bridge, and as aU 
determinations of the calorimeter coil resistance were made with the 1000 (right) and 
10 (left) as the arms, it is here only necessary to give the correction for these coils. 
Now 10L/1000R= 10-0077/1000-30 = -0100047. This we termed the “bridge 
correction.” 
The resulting values are expressed in legal ohms, and true ohms = reading in legal 
ohms(l — -0024275). 
By applying the corrections in the above form it was possible to perform all 
the operations by means of the slide rule,t as it was only necessary to detei-mine 
the quantity to be added or subtracted in one case to three, and in the other to 
four figures. 
(When working with platinum thermometers the last two corrections are omitted, 
as it is only necessary that the resistances should be expressed in terms of the same 
unit, the absolute value of that unit being unimportant). 
Table XII. gives the values of B, when the calorimeter coil was in water, stirred in 
the same way as daring a “ J ” determination. Each number in the column II was 
obtained in the manner given in full on p. 399, and includes the correction for tempera¬ 
ture. The “ plug correction ” cannot be deduced from column R, because the differ¬ 
ence between the correction on E, -h r and r has to be used in each case. The plug 
correction however is, in every case, about the same as that required by the values 
of R -|- n and r as given in .Table IV. 
* The correction for the temperatnre of the box was made before applying the “ plug con’ection.” The 
values of the temperatui’e coefficient for each dial had been previously determined b}’ Messrs. Elliott, 
and the accuracy of the coefficients has been exposed to severe tests. 
As the temperatnre of the box was controlled by a special regulator, it never differed greatly from 
17° C. (See ‘ Phil. Trans.,’ vol. 182, A, p. 45.) 
+ Fuller’s spiral slide rule gives results correct to 1 in 20,000 or 30,000. 
