Miscellaneous Intelligence. 173 



SCIENTIFIC INTELLIGENCE. 



1. Magnetic effect of Electrical Convection. — The magnetic 

 effect produced by the motion of an electrified body, first proved 

 by Rowland in 1876, and, though questioned by Cremieu, since 

 then repeatedly confirmed (see the article by Edwin P. Adams in 

 the present number, pp. 155-167, of this Journal), has been also 

 established by Harold Pender at the Johns Hopkins Physical 

 Laboratory. An account of his experiments is given in a recent 

 issue of the Johns Hopkins University Circular (No. 152, May- 

 June, 1901). The method of Cremieu, which, however, gave him 

 negative results, was employed. To produce the convection, two 

 micanite discs (diam. 30 cm.) gilded on both sides and charged 

 from a Voss machine and battery of six gallon Leyden jars, were 

 driven at a speed of 75 to 100 revolutions per second. An inter- 

 rupter made it possible to reverse the charge 12 to 25 times per 

 second. Earth-connected condensing plates were fixed opposite 

 each face of each disc and one centimeter distant. Between the 

 two inside condensing plates was suspended a coil of 1295 turns 

 of No. 21 copper wire connected through a commutating device 

 with an extremely delicate astatic galvanometer. The coil, cir- 

 cuit, and galvanometer were enclosed in earth-connected metallic 

 shields. The arrangement of the commutator was such that the 

 alternating current induced in the coil by the reversal of charge 

 in the rotating discs gave a steady deflection of the galvanometer. 



The apparatus was used in two ways: (a) the two discs were 

 rotated in the same direction and at any instant charged alike; 

 and (b) the discs were rotated in opposite directions and charged 

 oppositely at any instant ; of these, the second method gave 

 steadier deflections and was used more frequently. The direction 

 of deflection, repeatedly tested, was found, as expected, to be 

 always in accordance with Ampere's rule, i. e., the' motion of a 

 positive charge always produced the same effect as that of a con- 

 duction current flowing in the direction of motion of the charge. 



The quantitative results are discussed, as follows : " The 

 strength of a convection-current is defined as the quantity of 

 electricity carried convectively past any point in unit time. 

 On the assumption that a convection-current is magnetically 

 equivalent to a conduction-current of the same strength, the 

 current induced in the suspended coil on reversing the sign of 

 electrification of the discs can be readily calculated from the 

 dimensions of the apparatus and the difference of potential 

 between the discs and plates. This calculated value of the 

 current can then be compared with the observed value as deduced 

 from the deflection of the galvanometer. The formula for the 

 calculated value of the current involves the ratio of the two 

 systems of electric units, so that instead of comparing directly 

 the observed and calculated values of the current the two can be 

 equated and the value of the ratio v thus determined. The value 

 of this ratio thus found is a test for the accuracy of the assump- 

 tion that a convection-current is equivalent magnetically to a 



