236 Intelligence and Miscellaneous Articles. 



the circumference), the small difference bet-ween the convection 1 

 and the conduction -J- in general could not (or at least not with 

 certainty) be detected, that therefore, with approximate equality 

 of convection and conduction, the electrodynamic effect of the one 

 sensibly neutralized that of the other. 



When, however, the sector took in half of the circumference, the 

 here assumed current could be observed even in the free portion 

 of the disk, though the amount was too small for safe measure- 

 ment. 



The observed electrodynamic action being so little in the fore- 

 going experiments, in which the disk was electrified and covered in 

 its whole extent by the induced plates, theoretical calculation of 

 the amount of the action from the known absolute values of the 

 electrodynamic constants promised only approximately accordant 

 values. Nevertheless it was carried out by Mr. Rowland. 



The proportion in which the action of the earth's magnetism upon 

 the pair of astatic needles was diminished was ascertained by finding 

 the oscillation-period, first with the needles equally directed, and 

 then arranged astatically. 



The value of the function of the electric potential in the Leyden 

 battery, and on the rotating disk, was determined according to the 

 law of spark-length given by Sir William Thomson, which in this 

 case appeared sufficiently accurate. Before and after each experi- 

 ment, a smaller jar was charged from the battery of nine large 

 ones containing the store of electricity, and on it the length of the 

 spark was determined. 



The velocity of the rotation was regulated by the position of the 

 balls of a centrifugal governor, which was applied to one of the more 

 slowly rotating axes. The calculation from the magnitude of the 

 rollers agreed well with the determination by the tone of a siren- 

 disk, which was for some time attached i:o the most rapidly re- 

 volving axis. 



In the calculation of the distribution of the electricity on the 

 disk, and its electromagnetic directing-force, the surplus charge 

 present at the margin of the disk was reckoned according to the 

 value which holds for infinitely thin disks, and considered as an 

 infinitely thin thread concentrated at the margin — a proceeding 

 which was in both ways only approximately correct, but, in presence 

 of the minuteness of this portion, was sufficient. 



The influence exerted upon the upper needle was about ^ of 

 that upon the lower. The horizontal force of the earth's mag- 

 netism was put equal to 0*182 (using the centimetre, gramme, and 

 second as units) ; the electrodynamic constant was put by Mr. Row- 

 land, after Maxwell's determinations, equal to 28,80*0 millions. 

 W. Weber's value would be 31,074 millions. I give below under 

 M. the results calculated with the former value, under W. those 

 calculated with the latter. 



The following is the result of the calculation of only three series 

 of experiments conducted under favourable circumstances : — 



