icith Disks of Ebonite. 351 



Results obtained with the machine. — In the apparatus here 

 described, the dimensions are not very great ; the fixed disk 

 has a diameter of 35 centims., and the two rotating ones 30 

 centims. Between the electrodes (when the negative conductor 

 terminates in a spheroid) a stream of sparks is produced, with- 

 out using the condensers, at a distance of 6 centims. ; in the 

 opposite case, of 3 centims. With the condensers applied 

 (which have a tinfoil armature of 90 centims. square) numerous 

 sparks pass from one to the other electrode at a distance of 10 

 or 12 centims ; and in the most favourable circumstances (for 

 instance, when a slowly burning coal fire is at hand) the elec- 

 tricities are discharged even at a distance of 17 centims, — that 

 is, 2 centims. more than the radius of the rotating disk. This 

 occurred especially when the loss of electricity by streaming 

 out from the sharp points of the positive electrode was dimi- 

 nished by bringing it into contact with a small disk of ebonite, 

 fixed on the board near e or /. I must add here, as an impor- 

 tant property of this kind of electromachines, that the re- 

 versing of the electric condition of the paper armatures (which 

 is very inconvenient in the single apparatus) never occurs in 

 the double system. The only way to change the polarity of 

 the electrodes is first to discharge the machine entirely, and 

 then to excite again in the opposite manner. For the first- 

 described machine the polarity could be reversed by moving 

 the diametral conductor out of its position, and then bringing 

 it back again ; then the armatures always changed their elec- 

 tric condition. 



III. New Observations with Ebonite Machines. 



The properties of the diametral conductor. — The great dif- 

 ference between the electromachine with glass disks and 

 those with ebonite ones, consists in the peculiarity that the 

 latter absolutely require the diametral or inclined conductor, 

 in order to become active and to remain so ; by taking it away 

 the machine is immediately rendered inactive. 



On the contrary, it is sufficiently known that machines with 

 glass disks may be used without the conductor, although they 

 do not work as regularly, nor is the length of the spark as 

 great, as when provided with it*. It has been stated by 

 Prof. Poggendorff, that when no large paper armatures are 

 used it is necessary, in machines with glass disks, to take away 

 the diametral conductor ; otherwise the machine cannot be 

 excited. Now here it is quite the contrary : several times I 



* Experimenters with the machines with glass disks will have had 

 many occasions to observe that the length of the spark obtained without 

 this conductor can "be, with proper dimensions, 12 or 15 centims. 



