ELECTROMAGNETIC EFFECT OF COXVECTION-CUKRENTS 261 



again any deflexion on reversing the charge, although the precaution 

 was taken of testing it every day or so. 



The currents induced in the axle by the rotation caused no incon- 

 venience; if the disks are rotated in the same direction their effect is 

 added, while the effect of the axles is in opposite directions. Even 

 when the disks were rotated oppositely, the deflexion due to the axles 

 was only 3 or 4 cm., and remained perfectly constant. 



On running the disks, unelectrified, without the glass plates between 

 them and the needle, a deflexion of 4 or 5 cm. was noticed. This was 

 perfectly steady deflexion, and could easily be shown to be due to the 

 presence of the plate, as it ceased when the plates were replaced. 



This was very troublesome for a time, especially as the presence of a 

 brass plate in place of the glass was found to diminish the deflexion, 

 but did not bring the needle back to zero as the glasses did. On look- 

 ing at the figure (Plate Y, Fig. 1) it will be seen that there is a brass 

 plug (/) closing the bottom of the tube in which the needle is placed. 

 The rapid rotation of the disks caused a very appreciable exhaustion 

 at the centre, and consequently a steady stream of air was sucked down 

 the tube through the open mouthpiece, and out through the imperfect 

 connection of the plug. Air-currents were not at first suspected, as the 

 deflexion was so very steady. The brass plate used was smaller than 

 the glass, and hence did not completely shield the tube. 



After the brass back-pieces (H H) had been taken out, and a hard 

 rubber substituted, it was found that with one direction of rotation the 

 needle was extremely unsteady; it would run up the scale for several 

 centimetres, stop suddenly, &c. evidently a forced vibration. This 

 was traced to air-currents also. Now, the air blew into the open mouth 

 of the cone. The apparatus had been run for some months with this 

 open, and not the slightest irregularity had been seen. But the hard 

 rubber pieces were very much larger than the brass ones which were 

 removed ; they filled up the lower space to a greater extent, and deflected 

 the air upwards more than before, causing the unsteadiness. With the 

 opposite rotation the air was thrown down instead of up, and conse- 

 quently did not affect the needle. 



The first systematic observations were made in January, 1889, with 

 the disks charged and plates earthed. The deflexion on reversing was 

 got without difficulty, and it was in the direction to be expected; that 

 is, with positive electrification, the effect was equivalent to a current in 

 the direction of motion of the disk. A number of series were taken in 

 the next two months; they agreed among themselves well enough, but 



