114 Mi;. F. F, SMITH ON THE CONSTRUCTION OF 



It is evident from these observations that a difference of '01 O v per cent, exists ; or, 

 to express the difference fully, 



Resistance of Reichsanstalt unit Resistance of N.P.L. unit = : '00010 t ohm. 

 The eleven mercury standards constructed show that 



Resistance of international ohm Resistance of N.P.L. unit = '00008 5 ohm. 

 Hence 



Resistance of unit employed at the"! r Resistance of unit employed at the 

 Reichsanstalt as 1 international > < National Physical Laboratory as 



ohm I I international ohm 



= 'OOOOlg ohm. 



So that an approximate difference of two parts in one hundred thousand exists 

 between the units derived at the two laboratories. 



Assuming that all the changes in the B.A. coils have been rightly interpreted, and 

 that the correct ratio of the B.A. unit to the absolute ohm was fixed upon (see p. 1 15), 

 then for N.P.L. unit absolute ohm may be written. Hence, from the observations at 

 Teddington, 



Resistance of international unit Resistance of absolute unit = '000085 ohm. 



In other words, the absolute ohm is represented by a column of mercury 106 '292 

 centims. long, of 14'4510 grammes mass, at a temperature of 0'0 C. 



The ratio of the B.A. unit to the absolute ohm was, however, fixed to '01 per cent, 

 only, so that the above figures merely indicate the order of the difference. The relation 

 of the two units is, however, of interest ; more especially so, since it will shortly be 

 possible to again determine the coils in absolute measure. 



A comparison with the results obtained by Dr. GLAZEBROOK (' Phil. Trans.,' A, 

 1888) for the specific resistance of mercury is also possible. 



The specific resistance was determined in terms of the B.A. unit, the coils employed 

 being those known as Flat, F, G, and H, and belonging to the Association. These 

 four coils are now in the possession of the National Physical Laboratory, and although 

 considerable changes in the values of the coils have manifested themselves since 1888, 

 there is much evidence for the belief that the variations have been successfully 

 interpreted ('B.A. Report,' Southport, 1903). 



If it be assumed that the ratio of the B.A. unit to the absohite ohm is known, these 

 coils enable absolute measurements of resistance to be made. On such an assumption, 

 the resistances of the mercury standards dealt with in this paper have been deter- 

 mined. Similarly (but assuming a different ratio) Dr. GLAZEBROOK estimated the 

 specific resistance of mercury in absolute measure. If the changes in value of the 



