474 PROFESSOR W. E. AYKTON, MR. T. MATHER AND MR. F. E. SMITH : 



R, R, R, fig. 7, project upwards from the plate p', and from the middle and upper ends 

 of these rods sector-shaped pieces q project inwards and form tripods on which the 

 weights may rest. Claw-shaped lifters on the arms A, A, figs. 6 and 7, are operated by 

 cams C fixed in the corners of the balance case, and serve to remove or replace the 

 weights. The arrangement is very convenient, and works with perfect smoothness, 

 the result of the excellent workmanship of Mr. OERTLING. Two weights and two 

 lifters are provided at each end of the balance. These may be seen in the end view 

 of the instrument shown in fig. 7a, Plate 8, and also in fig. 6. 



Another novel feature of the balance is the arrangement employed for taking the 

 beam off the centre agate plane and fixing it in the zero position without appreciably 

 raising or lowering the suspended cylinders. This is of considerable importance, as it 

 allows of the coils being levelled and adjusted vertically to the sighted position, 

 without continually clamping and freeing of the beam for making and testing the 

 adjustment. The specification for the balance stated that " the displacement of the 

 suspended coils caused by fixing the beam must not exceed 4 mils (one-tenth of a 

 millimetre)." Mr. OERTLING has, however, used a construction which reduces the 

 displacement to a far lower figure, as the fixing is effected without raising the beam 

 more than 0'004 millim. (4 microns), and the planes carrying the suspended coils are 

 clamped with a movement less than 0'08 millim. 



The handle seen at the front of the case in figs. 2 and 6 actuates the clamping 

 arrangements. Turning it clockwise through 180 from the position shown lowers 

 the whole clamping beam F, fig. 6, thus bringing the centre knife edge against its 

 plane, and allowing the planes supporting the scale pans and cylinders to rest on the 

 end knife edges. By sliding inwards a tube surrounding, the horizontal clamping 

 axle, and turning the handle through another 90, two agate hemispheres m, m are 

 brought into contact with agate planes on the beam immediately above them and fix 

 the beam in the zero position whilst the scale pans and cylinders still hang on the 

 beam. This device is made use of when changing the weights, and on reversal of 

 current in making measurements. 



For observing the rest-point of the balance a microscope, seen at M a , fig. 6, is used 

 to view a finely divided ivory scale carried by the pointer at a distance of 14f inches 

 (37 '2 centims.) from the knife edge. The magnifying power of the microscope is 

 about 48, and the scale is f of an inch long, divided into 200 parts ; each division is 

 therefore ^ '' (0'095 millim.). The cross wires and the lines on the scale are 

 sufficiently fine to permit of one-twentieth of a division to be estimated quite easily, 

 and with care and practice it is possible to read to fiftieths of a division, and in some 

 cases to hundredths.* In all observations the method of vibrations was employed in 

 determining the rest-point, the amplitude being limited to a few divisions on either 

 side of the middle. 



* For illuminating the scale, a lens and a Nernst lamp placed some 6 feet away were used, and proved 

 most satisfactory. 



