354 MESSRS. R. T. GLAZEBROOK AND T. C. KIT/I'M KICK 



so that, if we write p. for 



we obtain 



~ W* I 1 + L (y^" 1 )} (4) 



and 



r = 



The term depending oil (L l)fL is, of course, extremely small, but in some of the 

 tubes employed it exercised a sensible effect on the result. 



In addition to the corrections necessary to reduce the results to the standard 

 temperature C., the lengths L and I require corrections of importance. 



The extremities of the tube opened into two large ebonite cups which were filled 

 with mercury, and the observed resistance R includes that of the mercury in these 

 cups which is situated just beyond the ends of the tube. Lord RAYLEIGH has shown 

 that, on the assumption that the diameter of the mercury column in the cups is 

 infinitely large compared with that of the tubes, the correction required would be 

 equivalent to adding to the length of the tube '82 of the diameter. The experiments 

 of MASCART, NERVILLE, and BENOTT, ' Re'sume' des Experiences sur la Determination 

 do 1'Ohm,' Paris, Gauthier-Villars, 1884, have justified this theoretical conclusion. 



To make more certain on this point, we decided to repeat MASCART'S experiments. 

 A third cup was made, of the same size as the two terminals which are described later, 

 differing from them only in having two openings, such as c, Plate 19, fig. 2. A tube, 

 95 '8 cm. long, and about 1 B.A. unit in resistance, was taken. Its resistance when 

 filled with mercury in the usual way was found. Its diameter, as found from its length 

 and resistance, was 1'08 mm. ; and therefore the theoreticfil correction for the two 

 ends is equivalent to an addition to the length of '886 mm., or to the resistance of 

 '00092 B.A. unit. The tube was then cut in two pieces, and an end was inserted in 

 each of the two openings in the third mercury cup. This was filled with mercury, 

 corked up, a thermometer passing through the cork, and replaced in the trough with 

 the usual packing of ice. When the whole had cooled down its resistance was again 

 measured and found to have increased by '00089 B.A. unit. This increase is due to 

 the resistance of the two cut ends, and the difference in the observed and the theoretical 

 values is within the errors of the determination. 



Another determination was made with a tube of about twice the cross section. In 

 this case the theoretical correction was equivalent to an increase of resistance of 

 '00061 B.A. unit, while the observed was '00059. It will be noticed that in both 

 cases the observed value was less than the theoretical. This result was also found to 

 be the case by LORENZ, who gives as the factor in the correction deduced from his 

 experiments, the value '82 '35 d-Jd^, d being the diameter of the bore, and <!., the 



