[ 661 ] 
XIII. Experiments to determine the Value of the British Association Unit of Resistance 
in Absolute Measure. 
By Lord Bayleigh, F.R.S. , Professor of Experimental Physics in the 
University of Cambridge. 
Received February 15,—Read March 9, 1882. 
[Plate 48.] 
The present paper relates to the same subject as that entitled “On the Determination 
of the Ohm in Absolute Measure,” communicated to the Society by Dr. Schuster and 
myself, and published in the Proceedings for April 12, 1881—referred to in the sequel 
as the former paper. The title has been altered to bring it into agreement with the 
resolutions of the Paris Electrical Congress, who decided that the ohm was to mean in 
future the absolute unit (10 9 C.G.S.), and not, as has usually been the intention, the 
unit issued by the Committee of the British Association, called for brevity the B.A. 
unit. Much that was said in the former paper applies equally to the present ex¬ 
periments, and will not in general be repeated, except for correction or additional 
emphasis. 
The new apparatus (Plate 48) was constructed by Messrs. Elliott on the same 
general plan as that employed by the original Committee, the principal difference 
being an enlargement of the linear dimensions in the ratio of about 3 : 2. The frame 
by which the revolving parts are supported is provided with insulating pieces to 
prevent the formation of induced electric currents, and more space is allowed than 
before between the frame and those parts of the ring which most nearly approach it 
during the revolution. It is supported on three levelling screws, and is clamped by 
bolts and nuts to the stone table upon which it stands. The ring is firmly fastened by 
nuts to two gun-metal pieces which penetrate it at the ends of the vertical diameter, 
and which form the shaft on which it rotates. The lower end of the bottom piece is 
rounded, and bears upon a plate of agate, on which the weight of the revolving parts 
is taken. A little above this comes the driving pulley (9 inches in diameter), and 
above this again the screw and nut by which the divided card is held. The top piece 
is hollow, forming a tube with an aperture of lj inches, and is held by a well-fitting 
brass collar attached to the upper part of the frame. On this bearing the force is very 
small, so that the considerable relative velocity of the sliding surfaces has no ill effect. 
Notwithstanding its great weight, the ring ran very lightly, and the principal resistance 
to be overcome was that due to setting air in motion. 
