78 MR. F. E. SMITH ON THE CONSTRUCTION OF 



tube rested were lined with felt, the tube being quite free and capable of vibration as 

 a whole. A gentle pressure of the tube against the disc commenced the grinding, a 

 slight travelling motion of the motor being at the same time maintained. The wax 

 prevented, to some extent, the detaching of tiny splints of glass from the interior of 

 the tube, and also blocked the way to grains of emery, which are liable to scratch the 

 inner skin of the tube and promote rupture. When sufficient grinding had resulted 

 for both ends to present an even appearance, and the measured length tallied 

 approximately with that desired, a slight convexity, was given to the ends by 

 deflecting the line of motion of the motor 1 to 1^, the tube being continually 

 rotated. Also, owing to a slight splintering of the outer glass, a fine bevelled edge, 

 inclined 20 to the ends, was ground. The final polish was produced by well-worn 

 emery of the finest kind. Examination with a lens indicated that such a surface was 

 very free from pittings and preferable to that obtained when the final polish was 

 made with a copper or a brass disc. Throughout these grinding processes a lubricant 

 of camphor and turpentine was used. 



One of the methods adopted for measuring the resistance of the standards (see p. 81) 

 requires the attachment of ebonite collars to the tubes. For this purpose three 

 equidistant dimples were made at a distance of 1 - 3 centims. from each end. These 

 were ground into the glass by a small emery wheel, the motor armature rotating 

 slowly, and a gentle pressure maintained. The apparatus is depicted in the upper part 

 of fig. 5. 



With the measurement of each standard's length the mechanical constants of the 

 tubes were completely known. Mr. KEELING determined these lengths (excepting 

 those of G and S) in the comparator bath ; his report is appended. 



Determination of Lj (Mr. B. F. E. KEELING). 



The tubes, after they were cut and ground, were again strapped to the brass bar, 

 holes being previously drilled in the bar so as to come directly under the ends of the 

 tubes. 



Two distinct measurements were made, the tube being turned through 180 between 

 them. The microscopes were focussed directly on to the ends of the tubes, no contact 

 pieces being employed. No difficulty in focussing was experienced. 



