ON STANDARDS OF ELECTRICAL RESISTANCE. 



483 



of measuring an infinitely small or infinitely large resistance ; but clearly the 

 resistance of G + r shoiid be so arranged in each case that the angle ob- 

 served was not very diffez'ent from 45°. The range of the instrument may 

 be further increased by the use of elements. 



III. Comparison of B.A. Units to he deposited at Keiu Ohsei'vatory . 



By C. Hockijst. 

 The following Table shows the value of the cUfferent copies of the B.A. 

 units that have been made for preservation at Kew : — 



* The alteration of this coil, observed on February 11, 1867, is due, no doubt, to a defect 

 observed in the glass tube. 



The tube was of lead-glass. Perhaps the strong nitric acid used to clean the tube 

 attacked the glass. A new mercury unit (Xo. III.) was made in consequence of this 

 defect. 



The apparent alteration in the platinum-iridium coils from tlie first value found, I believe 

 to be owing to a clerical error. No alteration has been observed in them since the second 

 observation made by Dr. Matthiessen in June 1865. 



The values given 'in the above Table are deduced from the German-silver coil called B, 

 used in your Committee's experiments in 1SG4. This coil was found (by comparison with 

 copies made in 1864, of gold-silver, German silver, and platinum silver) not to have altered. 

 The coil B was also compared with the coil (June 4) used in 1863, and the ratio of the 

 two coils was fomid not to liave altered. 



lY. Experiments on Capacity. By Fleejiixg Jenkii^, F.B.S. 

 The capacity of a condenser made of mica and tinfoil was adjusted so as 

 to be approximately equal to lO-i^ electromagnetic absolute tmits, according 



2k2 



