76 MR. F. R SMITH ON THE O INSTRUCTION OF 



The standards S and G stand apart from the rest. These two were not measured 

 by Mr. KEELING owing to the impossibility at that time of measuring any length 

 greater than 110 centims. in the comparator bath. The method adopted with these 

 was very similar to that employed by Mr. GLAZEBKOOK in 1888. 



The tubes were filled as before, but for length determinations, since liquid immersion 

 was out of the question, the temperature was necessarily somewhat uncertain. The 

 observations were made in the Electrical Standards Room, which, as before mentioned, 

 is maintained at a constant temperature. Subject to the condition of a constant 

 barometric pressure, the temperature of this room may be kept steady to 0'1 C. 

 Here the tubes were placed in position, and every preparation made for the length 

 determination, the comparator by PYE being employed. The mercury column having 

 been adjusted, the menisci were focussed, and the arrangement left undisturbed over 

 night, an open scale thermometer lying alongside the tube for temperature observa- 

 tions. Under favourable conditions, the temperature chart of this room for the night 

 interval is a perfectly horizontal line, so that no greater error than "002 per cent, 

 (corresponding to 0'1 C.) was anticipated. Immediately on entering the room next 

 morning, the temperature as indicated by the thermometer was read, the microscope 

 slightly adjusted and the readings taken, the whole occupying about 1 minute. The 

 standards of length were then substituted and the measurements rendered complete. 

 The variations shown in Table III. are greater than expected, and the weight given 

 to these two standards must necessarily be less than that given to the others. 



Apart from the tubes S and G, the greatest difference from the mean is, in three 

 cases -only, greater than O'OOl per cent. Irrespective of sign, the mean difference is 

 (V0006 per cent., or six parts in a million. 



TJ>e Cutting and Grinding of the Tubes. 



The desired lengths and the positions of the extremities being known, the tubes 

 had now to be cut. 



To do this, a file cut was made in each case about 0'5 millim. distant from the 

 point desired, so that, when broken, the standard portion was about a millimetre 

 longer than requisite. This was advisable, since, on breaking, tiny splints of glass 

 separated from the interior of the tube, making the cross-section there greater than 

 before. The ends of the standard were next warmed and immersed in molten wax ; 

 on removing, the wax solidified, forming a solid plug about 3 centims. long at each 

 end. Paraffin wax was at first tried, but melted too readily. A mixture of paraffin 

 wax and beeswax in the proportion of three to one was eventually used and answered 

 capitally. 



For grinding the ends, a slide rest of one of the lathes of the laboratory was so 

 adapted as to secure a small electric motor firmly in position on its upper surface. 

 Originally it was desired that the ends of each tube should be perfectly plane surfaces 



