the Mercury Unit ivith the British Association Unit. 21 



or four and a half hours. The boards were therefore replaced 

 by narrow partition-blocks, scored to allow the tubes to rest 

 firmly. The tubes were thus raised about three quarters of 

 an inch from the bottom of the box. 



Most observers have measured the length of the column of 

 mercury, used in determining the cross section of a tube, at 

 the temperature of the room (between 10° and 20° C), and 

 then used a formula which reduces their observations to 0° G. 

 As the cubical coefficient of expansion of mercury in glass 

 is '00016, an error of little more than six tenths of a degree 

 will make a difference of one part in ten thousand in the final 

 result. Since the mercury- column is in a thick-walled glass 

 tube, simply exposed to the air of the room (generally for a 

 few hours), the uncertainty of its being at the temperature 

 shown by thermometers placed alongside the tube may be 

 readily seen. The tendency of this error will be to give too 

 high a value for r. Grlazebrook and Fitzpatrick measured the 

 length of the column at intervals of fifteen minutes ; and when 

 two consecutive readings coincided, it was assumed that the 

 mercury was at the temperature shown by the thermometers. 

 They verified the result in several cases after the mercury had 

 been blown out into a small capsule, but do not mention how 

 they measured accurately the temperature of so small a volume 

 of mercury. 



In view of the results of our preliminary observations it 

 was decided to determine the mean cross sections at zero, by 

 using the mercury upon which the resistance-measurements 

 had been made and obtaining a full tube as follows : — 



When through with the resistance -measurements, one end- 

 piece was removed and the tube stopped by one finger, over 

 which was a tight, elastic, pure gum-band. The other end of 

 the trough was then raised to an angle of about 20° without 

 disturbing the tube in the ice, the end-piece quickly slipped off, 

 the end of the mercury-column flattened off with a similarly 

 covered finger, and any globules wiped away. The angle of 

 the box being reversed, the mercury was allowed to flow out 

 into a watch-glass, being afterwards dried over pumice-stone 

 soaked with strong sulphuric acid. 



Supposing that the exposed ends (about 6 centim. in all) 

 rose to an average of o° C, which they could hardly do in the 

 few minutes necessary to empty the tube, as they were in such 

 close proximity to the ice, and the original temperature less 

 than o, 3C.,the error due to this cause would make the result 

 three parts in one hundred thousand too low. 



Determination of \x. — The tubes were furnished by Eimer 

 and Amend, of New York, and out of a very large number 



