SOME MI i;cri:v ST\\M.M;[>S OF I;KSISTA\CK, KTC. 71 



sensitiveness was, however, increased l>eyond this, since small loads only were 

 employed and stability could be ensured. In addition, the readings of the zero were 

 rendered more accurate by avoiding parallax, and by optically magnifying the 

 motion of the beam. These latter refinements were secured by fixing a plane mirror 

 at the centre of the l>eam, the axis of oscillation of the mirror being in line with the 

 knife edge of the central agate. A beam of light from an incandescent lamp first 

 passed through a converging lens, and was then reflected from the mirror to a 

 transparent scale, situated about 3 feet distant from the mirror. An adjustment of 

 the position of the lens enabled a sharp image of the filament to l>e formed on the 

 screen, and, since the axis of the optic beam lay in the horizontal plane containing 

 the knife edge of the balance, small displacements of the zero were proportional to 

 the disturbing forces. In this way the reading of the luilance zero was a pleasure. 

 In practice a change in zero of two scale divisions (1 scale div. = I millim.) 

 corresponded to a hundredth of a milligramme. 



After the length of the mercury column had l>een measured in the comparator 

 bath, the capped tul)e was dried on the outside and left in a comparatively warm 

 room for an hour or two hours. In some cases the tul>e was slightly warmed by a 

 Bunsen flame held Inflow and at a considerable distance from the tube. This was to 

 ensure that the temj>erature of the mercury should l>e at least equal to that of the 

 room, or otherwise deposition of moisture would produce trouble. The rubber caps 

 were removed, and after examining the ends of the tulx? for moisture, the mercury 

 was quickly run into a small weighed watch ghiss and removed to the lialance case. 

 The temperature of the latter was approximately constant, being situated in a 

 constant-temperature room. In every case, however, the temj>erature was read 

 together with the lirometric pressure. Weighings were conducted on lx>th pans 

 of the Ijalance, and the sensitivity frequently determined. The latter, however, was 

 very constant indeed. 



Calculations and Explanation of Tabln III. (pp. 74. 75) 



It follows from the equations on p. 58 that the condition for a mercury column at 

 C. to have a resistance of one international ohm is that its length shall be equal 

 to 106-3 2 7rr 8 A/]4-4521/*= 781-8727ir s A//i, where p. is the conical correction of the 

 column, its mean radius, and A the density of mercury at C. Since the 

 calculated cross-section is an inverse function of the density of mercury, the length 

 L is independent of the density. The value assumed for the latter in the calculations 

 will be found to be afterwards eliminated, but a knowledge of the coefficient of 

 cubical expansion for mercury is assumed. The coefficient given in LANDOLDT'S 

 Tables has l>een employed. 



The approximate coefficients of expansion for Jena and Verre dur glass have been 



