106 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
The resistances which we have employed have been compared with nine new mercury 
standards of resistance constructed in accordance with the Specifications of the London 
Conference on Electrical Units and Standards (1908). These mercury standards of 
resistance practically realise the international ohm then defined, and the agreement 
between the nine standards is very good. # There are, however, certain sources of 
error in the construction of such standards which must always be of the same sign, and 
the probable error associated with the practical realisation of the international ohm 
has been estimated to be not less than 2 or 3 parts in 100,000. 
The mean of the results given in Tables XX., XXI., XXII., and XXIII. leads to 
the conclusion :—- 
A resistance of 1 international ohm is equal to 1’00052 ±Q’00Q04 ohm (10 9 cm./sec.), 
the probable error ±0’00004 being approximately the sum of those involved in the 
realisation of the ohm and the international ohm. 
The international ohm, as defined by the London Conference on Electrical Units and 
Standards (1908), is the resistance at 0° C. of a column of mercury, 14'4521 gr. in mass 
of a constant cross-sectional area and of a length of 106"300 cm. As stated in the 
Introduction, the cross-section of such a column is equal to 1 sq. mm. or nearly so. 
Since the international ohm is equal to 1‘00052+ 0‘00004 ohms, the mass of the 
column of mercury of the same cross-sectional area as the international ohm and having 
a resistance of 1 ohm will be - 14 45^1 -_ ^ 4-4446 + 0‘0006gr.,t while the length 
1‘00052±0‘00004 13 
of the column will be- 106 300 - _ 105-245 + 0'00004 cm.t 
1‘00052±0‘00004 
We may sum up our results by stating that:— 
The ohm 1G 9 cm./sec. is represented by the resistance at 0 C. of a column of mercury 
14’4446±0’0006 gr. in mass, of a constant cross-sectional area (the same as for the 
international ohm) and having a length of 106’245±0’004 cm. 
The Historical Introduction shows a number of determinations, notably those of 
Rayleigh (corrected values marked (S) Table I.), Glazebrook (corrected value (S) 
Table I.), Wiedemann, Dorn, and Himstedt, in close agreement with that now 
obtained. These results are as follows :— 
1882, Rayleigh. . 
1882, Glazebrook . 
1883, Rayleigh. . 
1885, Wiedemann . 
1889, Dorn . . . 
1892, Himstedt . . 
106‘26 
106‘25 
106‘24 
cm. 
; ? 
95 
106-265 „ 
106-243 „ 
106-259 „ 
* ‘Report of the National Physical Laboratory for 1912.’ 
t The 'probable errors in these two values are so related that an error in either value is necessarily 
associated with an equivalent proportionate error in the other. 
