418 Mr. C. F. Brush on the Measurement 



The most important part of the gauge is the head B. The 

 purpose of its great diameter is the reduction of capillary 

 depression in its mercury column. But its size necessitates a 

 very close approach of the mercury to its upper end, in order 

 to sufficiently reduce its capacity. Yet the remaining space 

 must be measurable by the cathetometer with the utmost pre- 

 cision. Hence the glass must not be distorted by heating, 

 and the closed end just over the mercury must be sharply 

 defined. In constructing this part of the apparatus, I selected 

 a piece of heavy tubing which would just slip inside of B, 

 with the least possible clearance. One end of this tube was 

 closed as squarely as possible by fusion, and then ground with 

 fine emery and a suitable tool to a convex spherical surface of 

 long radius. Care was taken to make the centre of curvature 

 lie in the axis of the tube, and the ground surface was left 

 unpolished to facilitate observation. A suitable length of the 

 closed end of the tube was then cut off, slipped into B, and 

 both tubes were fused together at their open ends, as shown. 



For calibrating the head B, a ground-glass stopper with a 

 capillary duct was fitted to its neck, before the latter was 

 sealed to the bulb A. The head was then filled with mercury 

 by boiling, thus completely filling the small space between its 

 wall and the cap. After cooling, the stopper was inserted to 

 expel all excess of mercury, and the whole weighed. Next the 

 head was emptied, and the mercury in the annular space dis- 

 tilled out. Again the head was very nearly filled with mercury, 

 without allowing any to get into the annular space, and weighed 

 as before ; and the space between the top of the mercury and the 

 convex end of the head was very carefully measured by the 

 cathetometer. This process of weighing and measuring was 

 repeated several times, with less mercury each time. Thus 

 the capacity of a vertical millimetre of the head was ascer- 

 tained, as well as the capacity that would remain, if the top 

 of the meniscus of mercury just touched the convex end of 

 the gauge above it. Finally the neck was sealed to the 

 bulb A, and the capacity of head, neck, and bulb combined 

 was found by weighing them empty, and again filled with 

 mercury. 



For lighting the top of each mercury column, a narrow 

 horizontal slit in an opaque screen B is used. The slit is 

 covered with a strip of ground glass, and obliquely illuminated 

 by an electric lamp. The screen and slit are vertically adjusted 

 by a thumb-screw S. The heat of the lamp is prevented 

 from reaching the mercury columns, and head B, by a thick 

 screen. This is very necessary. 



In order to get the best results from the apparatus, many 



