with Disks of Ebonite. 349 



is absolutely necessary ; and even when powerful sparks are 

 passing between the conductors, tbe electric manifestation dis- 

 appears immediately as soon as the inclined conductor is taken 

 away : the disks seem to have entirely lost their charge ; 

 and even in the dark not the slightest visible trace of electri- 

 city can be detected in the apparatus. The action is completely 

 resumed when its inclined conductor is again brought to the 

 armatures, even at a distance of 2 or 3 centims. The apparatus 

 may be excited also by placing for some moments a piece of 

 rubbed ebonite behind the rotating disk opposite to the points 

 of the inclined conductor. Much electricity, attracted by in- 

 duction, flows on this surface from the points ; and this soon 

 causes a stream of sparks to pass between the horizontal con- 

 ductors. It must be observed that the excitation is only ob- 

 tained when these conductors are not in contact ; yet this is 

 necessary when glass disks are used. I tried, but in vain, to 

 put the apparatus in action without the inclined conductor, or 

 with it when the horizontal conductors were brought close 

 together. 



Results obtained with the machine. — In the machine above 

 described I have used disks of the greatest diameter hitherto 

 employed. That of the fixed disk is 58 centims., and of the 

 rotating one 55 centims. The condensers in connexion with 

 the horizontal conductors are Leyden jars, with a covered sur- 

 face of 1 square decim. Without them, when the negative 

 conductor ends in a spheroid (9 centims. diameter) and the 

 positive in a sphere (2 centims. diameter), a continuous stream 

 of sparks passes at a distance of 4 centims. At a greater dis- 

 tance a beautiful sheaf appears between the conductors, con- 

 sisting of a stem of 2 or 3 centims. with a bundle of thin violet 

 threads which continue to radiate towards the opposite con- 

 ductor, even at a distance of 17 centims., neutralizing the nega- 

 tive electricity. When the condensers are connected, energetic 

 and very bright sparks are obtained, the greatest length of 

 which under favourable circumstances reach 25 to 26 centims. 

 But at the ordinary temperature, without special heating, the 

 length of the spark is 17 or 20 centims. ; and in a moist atmo- 

 sphere the machine never remained active ; even sparks of a 

 length of 10 centims. were produced. 



It may be concluded from these facts that the ebonite machine 

 equals the machine with glass disks ; besides, its power of re- 

 taining its electric charge is so high, that I often found it in 

 electric condition when four weeks had elapsed ; and the action 

 was entirely restored when the disk was set in rotation. It 

 was even somewhat difficult to put the machine in a wholly 

 neutral condition, if required for investigation ; a slight trace 



