tlie Maanetic Effect of Elect nc Convection. 457 



lience no deflexion. However, if in the galvanometer circuit 

 is placed an interrupter actuated by an eccentric on the axle 

 ot" the disk, arranged so as to close the galvanometer circuit 

 during only half a revolution of the disk, a uni-directed 

 current can be obtained. If the eccentric is adjusted so as 

 to close the galvanometer- circuit while CD turns from PR 

 to RP, the difference of potential between ( ' and D will pass 

 from to through a maximum corresponding to the moment 

 when CD is in the position .vi/. The current through the 

 galvanometer will then be uni-directed, and a deflexion 

 should be obtained. If the eccentric is shifted 90° so as to 

 close the galvanometer-circuit a quarter of a period later, 

 this deflexion should become nil, because the difference of 

 potential between C and D passes from the value —e to +^ 

 during the half period of contact. Between these two posi- 

 tions of the eccentric all intermediate deflexions should be 

 obtainable. 



This experiment proved unusually satisfactory. The 

 maximum deflexion was quite large and the measurements 

 agreed within 2 per cent, with the calculated values, which, 

 considering the numerous approximations which must neces- 

 sarily be made in determining the various constants (surface- 

 density of the charge, relative resistances of the various parts 

 of the circuit, speed of rotation), is all that could be desired. 

 From the results w^e may therefore conclude that : 



A cliavf/ed metallic surface moving in its own 2^l(^^^e in the 

 presence of ji.ved parallel metallic surfaces carries its charge 

 along icitli it. 



Edperiments with Sectored Disks. Cremieus '' Open Current " 

 Method. — Having thus solved in the affirmative the question 

 of the existence of a magnetic effect produced by the rotation 

 of charged continuous disks, and having verified directly the 

 entrainement of the charge, we returned to the experiments 

 made with sectored disks. 



During the past year, Cremieu * obtained with disks 

 formed of insulated sectors magnetic effects which were quite 

 irregular and without any quantitative relation to the intensity 

 of the convection-current, which, in these experiments, was 

 measured directly. The apparatus consisted of a core of 

 ebonite 24 cm. in diameter carrying 18 sectors of micanite 

 13 cm. long; separated from each other by 2 cm. of air. 

 Only the outer portion of the sectors was gilded, the gilt 

 covering a vvidth of f) cm. The moving sectors M (fig. 5) 

 passed between two charged fixed sectors S, and at the same 



* Comptes Rendus, vol. cxxxvi. p. 27 (1902). 



