On the Mercury Unit and the British Association Unit. 17 



gences of the gold leaves were determined by an absolute 

 electrometer, and the results are shown in fig. 1 (Plate I.). 



It is probable that this valuable property of quartz, that it 

 insulates perfectly in damp air, may be of use in the con- 

 struction of electrostatic apparatus. The sulphuric acid now 

 absolutely necessary in electrometers and instruments of that 

 class is nothing short of a nuisance. If the instrument is 

 carried about there is the risk of destruction of the instrument 

 from the spilling of the acid. If the instrument is not moved 

 the acid, unless specially treated, may give off nitrous fumes 

 which will corrode the surfaces of metal ; or, if forgotten, 

 it absorbs water and in time overflows, destroying the whole 

 apparatus. Even if the air were saturated with moisture, rods 

 of quartz would insulate as well as the lead -glass at present 

 used does in air dried by sulphuric acid. The needle should 

 of course be suspended by a fibre of quartz, which is far 

 simpler to apply and adjust than the double line of silk, and 

 superior also in other respects. 



In conclusion I must express my obligations to Mr. Briscoe, 

 a student in the laboratory, whose skill in the manipulation 

 of gold leaf and whose suggestions from time to time have 

 been of the greatest service. I have with perfect confidence 

 asked him to carry out the experiments described in this paper, 

 and the results show that the confidence was not misplaced. 



111. A Comparison of the Mercury Unit with the British Asso- 

 ciation Unit of Resistance. By Cary T. Hutchinson and 

 Gilbert Wilkes*. 



fTPHE object of this research, which was conducted in the 

 -*- Physical Laboratory of the Johns Hopkins University, 

 under the supervision of Professor Henry A. Rowland, is the 

 determination of the ratio of the resistance, at 0° C, of a 

 column of mercury, 1 metre long and 1 square millimetre in 

 cross section, to the British Association unit of electrical 

 resistance. 



The method employed in making the observations was, with 

 slight modifications, the same as that used by Lord Rayleigh, 

 by Glazebrook and Fitzpatrick, and also in a similar determi- 

 nation already made at this laboratory. 



The resistance at 0° C. of a column of mercury, filling a 

 fine, accurately calibrated glass tube, is determined in British 

 Association units ; the length L is known ; its mean cross 



* From the ' Johns Hopkins University Circular ' for May 1889. 

 Communicated by the Authors. 



Phil. Mag. S. 5. Vol. 28. No. 170. July 1889. C 



