452 



Messrs. V. Cremieu and H. Pender 



period of oscillation was about 60 seconds. When this system 

 was placed near the moving disk in a suitable screen, it was 

 absolutely unaffected by the starting of the motor and the disk. 



However, an unexpected effect prevented us from obtaining 

 what we had hoped. At the end of about two minutes' 

 rotation of the disk, over which the system was placed, the 

 system began slowly to deflect, the deflexion increasing as 

 the time went on, thus rendering all observations impossible. 

 At first w^e attributed this deflexion to air-currents set up in 

 the screen containing the system by the heating of the con- 

 densing-plate over the disk. This plate became quite warm 

 on account of the violent vibrations caused by the rapid 

 motion of the disk. But the " dummy " system with the 

 non-magnetic needle gave no such deflexion ; hence the 

 effect could not be attributed to air-currents. We finally 

 concluded that the cause of this effect was the following: — 

 The rotation of the disk immediately below the pole of the 

 magnet causes Foucault currents to be set up in the gilded 

 surface, and the reaction of these on the magnet produces 

 either a slow demagnetization or a change in the position of 

 the magnetic axis of the needle with respect to its geometric 

 axis. However, be the cause what it may, we were forced 

 to abandon the use of such systems. 



Returning to the ordinary systems, we constructed a very 

 sensitive one in the following manner : — Three pairs of very 

 fine steel needles, magnetized to saturation, were arranged 

 upon a thin sheet of mica as indicated in fig. 2. This system 

 was 6 cm. long and weighed about 500 mg. It was 

 suspended inside a metallic tube from a silk fibre 

 about a metre long. A test current-sheet con- 

 structed in such a manner as to give a distribution 

 of current similar to that which is produced by 

 the disk, was used to determine the sensibility of 

 the system. When this test-sheet was put in 

 place of the disk and a current of 10 ~^ ampere 

 sent through it, the system was deflected 120 mm. 

 This sensibility was amply sufficient, since the 

 convection-current realized in the experiments was 

 between 3 and 5 x 10~^ ampere. 



But even with this system we were not able 

 to realize exactly the experiment which we have 

 described. In fact, the micanite disks which we 

 employed are magnetic ; and very unequally so 

 at different points. Consequently the system 

 would take different positions of equilibrium de- 

 pending upon the part of the disk immediately 

 under it. However, if the disk is given a velocity such that 



Fio-. -2. 



