402 REPORT—1883. 
key, placed in any convenient position. The only precaution required is not to 
bring other magnets into the neighbourhood of the balance, or at any rate not to 
move them during a set of weighings. 
The other point as to which I wish to make a suggestion relates to the time of 
vibration of the beam. I think that, with the view of obtaining a high degree of 
sensitiveness, the vibrations are often made too slow. Now the limit of accuracy 
depends more upon the smallness of the force which can be relied upon to displace 
the beam in a definite manner than upon the magnitude of the displacement so 
produced. As in other instruments whose operation depends upon similar 
principles, e.g. galvanometers, it is useless to endeavour to increase the sensitiveness 
by too near an approach to instability, because the effect of casual disturbances is 
augmented in the same proportion as that of the forces to be estimated. If the 
time of vibration be halved, the displacement due to a small excess of weight is 
indeed reduced in the ratio of four to one, but it is not necessarily rendered any 
more uncertain. The mere diminution in the amount of displacement may be 
compensated by lengthening the pointer, or by optical magnification of its motions. 
By the method of mirror-reading such magnification may be pushed to almost any 
extent, but I am dealing at present only with an arrangement adapted for ordinary 
use. 
In the balance (by Oertling) that I am now using, the scale-divisions are 
finer than usual, and the motion of the pointer is magnified four or five times 
without the slightest inconvenience by a lens fixed in the proper position. The 
pointer being in the same plane as the scale-divisions, there is no sensible parallax. 
In this way the advantage of quick vibrations is combined with easy visibility of 
the motion due to the smallest weights appreciable by the balance. 
To illuminate the scale the image of a small and distant gas flame is thrown 
upon it by means of a large plate-glass lens. This artificial illumination is found 
to be very convenient, as the instrument stands at some distance from a window, 
but it is not at all called for in consequence of the use of the magnifying lens. 
FRIDAY, SEPTEMBER 21, 
The following Reports and Papers were read :— 
1. Report of the Committee for constructing and issuing practical Stan- 
dards for use in Electrical Measwrements——See Reports, p. 41. 
2. On a case of Rapid Diffusion of Molten Metals. By Professor 
CHANDLER Roserts, /.R.S. 
The question of the diffusion of metals, recently attacked by Dr. Guthrie at the 
ordinary temperature in the case of amalgams of mercury, has been examined by 
Professor W. Chandler Roberts, who has dealt with metals having a higher 
melting point than lead. He shows by a series of careful experiments that while 
molten copper and antimony interpenetrate but slowly, the ‘mobility’ of gold and 
silver in molten lead is comparatively rapid. Hxact numerical determinations of the 
rate of passage have yet to be made, but the velocity of gold and of silver in 
molten lead is so great as to resemble the diffusion of gases rather than the 
diffusion of a solid in a liquid. Professor Roberts has already extended his 
observations to silver and gold in molten bismuth, and, as Sir W. Thomson 
observed, the question is one of extreme importance and interest both to the 
physicist and metallurgist. 
3. On the Magnetic Susceptibility and Retentiveness of Soft Iron. 
By Professor J. A. Ewine, B.Sc., F.R.S.L. 
During three years the writer has been engaged, while in Japan, in prosecuting 
researches on the magnetisation of iron and steel, and on the effects of stress on 
