PHYSICS. . 501 



Edlnnd Las discussed the phenomena which bear on the question of 

 the electrical resistance of vacuum, and comes to the conclusion that 

 they all indicate that a vacuum is a good conductor of electricity, the 

 resistance commonly experienced taking place between the electrode 

 and the vacuum. " It seems to me," he says, " that in drawing, from 

 the known impossibility of an electric current traversing the most per- 

 fect vacuum between electrodes, the conclusion that a vacuum is abso- 

 lutely non-conducting, the same mistake has been made as when from 

 the circumstance that the sun rises in the east and sets in the west, it 

 was believed that one might infer that the sun in reality goes round 

 the earth." {Phil Mag., Y, xiii, p. 1,200 ; J. Phys., May, II, i, p. 235.) 



Rayleigh in conjunction with Mrs. Sidgwick, has determined the spe- 

 cific resistance of mercury. Four tubes were used to contain the mer- 

 cury, from 87 to 194 centimeters long. Tube I gave the value 0.95416, 

 tube II 0.95419, tube III 0.95416, and tube IV 0.95427. Hence the 

 mercury unit is 0.94130 x 10^ c. G. s. {Nature, May, xxvi, p. 94; J. 

 Phys., July, II, I, p. 327.) 



S. P. Thompson has made a series of measurements to prove that the 

 change in electric resistance produced by pressure on carbon is solely 

 due to an increase in the perfection of the superficial contact. The 

 carbon used was Carre's electric light carbon. {Phil. Mag., April, Y, 

 XIII, p. 262 ; Nature, March, XXV, p. 427.) 



Mendenhall, using the soft carbon buttons made for the Edison tele- 

 phone, finds not only that the resistance is diminished by pressure, but 

 also that this decrease in the resistance continues for some time. When, 

 for example, a carbon button having a resistance of 11.67 ohms was 

 pressed by a weight of 50 grams, this resistance fell at once to 3.52 

 ohms ; but it continued to fall for an hour and a half, and even for 

 24 hours. In one case pressure was continued for a week ; but uporl 

 removing the pressure the original resistance was at once recovered. 

 {Am. J. Sci., July, III, xxiv, p. 43.) 



4. Electric SparJc and Electric Light. 



Mpher has put into practice at the observatory of the Washington 

 University, Saint Louis, a simi:)le device for the transmission of clock- 

 beats upon telegraph lines. In it two platinum plates attached by a 

 light framework fastened to the lower end of the pendulum cut alter- 

 nately a globule of mercury just below them. As the break is deter- 

 mined by the distance apart of the plates, and as these are adjustable, 

 the length of the break may be made whatever is desirable. {Ain. J. Sci., 

 July, III, XXIV, 54.) 



Bailie, by means of a special apparatus, has measured the differences 

 of potential corresponding to a given distance of the explosive dis- 

 charge. He concludes, (1) for the same length of spark, the ])otential is 

 a maximum when the two electrodes have the same curvature ; (2) the 

 potential varies from the maximum in proportion as the difference be- 



