PHYSICS. 381 



changed b3^ displacing this plate ; and this without putting the needle 

 in a position of unstable equilibrium. Moreover, the influence of ter- 

 restrial magnetism on a galvanometer may be materially diminished by 

 surrounding it with a ring of iron. (Anz. Ak.Wien, 1885, 148; J. Phys., 

 May, 1886, II, v, 242.) 



Carhart has observed that the equation expressing the electrical en 

 ergy absorbed by a motor per second as the product of the counter-elec- 

 tromotive force and thecurrent Ei^ — EEj— -R W, is an equation of the 

 second degree, and therefore represents a conic section. On applying 

 the proper criterion he finds that the locus of this equation is a hyper- 

 bola. Plotting the curve, he draws important inferences from it, and 

 shows that Jacobi's law of maximum rate of working applies only under 

 the condition of a constant electromotive force. (Am. J. Sci., February, 

 1886, III, XXXI, 95.) 



Mendenhall has repeated and confirmed his experiments on the 

 diminution of the resistance of soft-carbon disks with pressure. With 

 a rod of hard carbon 12*"" long and 1.5'='" in diameter, with plated con- 

 tacts inside of the ends, he observed a decreased deflection whenever 

 the rod was compressed in a vise. The soft-carbon disk was made to 

 form a partition between the two halves of a (J tube, and, mercury 

 being poured in so as to entirely cover the surfaces of the carbon, the 

 ends of the tubes were closed. On then blowing in through a rubber 

 tube connected laterally to each leg pressure'could be produced on the 

 carbon disk and the effect on the current j)assing through the disk 

 noted. All the experiments without exception showed great diminu- 

 tion in the resistance of the disk by increase of pressure; this resist- 

 ance in one case falling to one-half when the pressure was increased by 

 S*"" of mercury. {Am. J. ScL, September, 1887, III, xxxii, 218.) Tom- 

 liuson has published a note on these experiments, stating that he pre- 

 fers to believe that the change does not take place in the specific resist- 

 ance of the carbon itself, but is due to better surface contact ; erro- 

 neously assuming that the Edison soft-carbon disks are formed by com- 

 pressing a mixture of lamp-black with gum-water. (Phil. Mag., No- 

 vember, 1886, V, XXII, 442.) 



* 4. Electric spark and electric light. 



According to the hypothesis of Edlund, a perfect vacuum is a good 

 conductor of electricity, and the resistance of a gaseous column more or 

 less rarefied is composed of two terms, one of which represents the re- 

 sistance proper of the gas which diminishes with the pressure, tbe 

 other a resistance of which the surface of the electrodes is the seat, and 

 which varies according to an entirely different law, increasing as the 

 density of the gas diminishes and becoming so great at the lowest 

 pressures that can be produced artificially as to have given the impres- 

 sion that a vacuum is an insulator. Hom^n, under Edlund's direction, 

 has made a series of measurements on the resistance of air columns 



