348 Dr. L. Bleekrode on Electromachines 



stance ; and, as far as I could consult different periodicals, I 

 have found no mention of a true Holtz machine with disks of 

 ebonite. Ebonite (a mechanical mixture of india-rubber and 

 sulphur) has many advantages for this purpose, which glass 

 can but rarely offer. It is capable of developing much more 

 electricity, and readily becomes electric ; it is much less hy- 

 groscopic, and, especially with regard to the boring of the 

 apertures in the disk, is cheaper. Here may also be added that 

 no part of the apparatus is liable to be broken. Yet unfavour- 

 able properties must be noticed, which are well known to 

 experimenters with this substance. It tends slowly to alter 

 its composition, especially if exposed to the solar rays or to the 

 action of ozone ; the sulphur is separated and transformed into 

 sulphuric acid, and the insulating properties are destroyed. 

 But it may be observed here that some qualities of ebonite are 

 the result of great negligence ; sometimes a bad kind of raw 

 material enters into its manufacture ; so that very different 

 sorts are found in commerce. When a demand shall have been 

 created for a quality of ebonite specially suitable for electrical 

 machines, it may be expected that more attention will be paid 

 to its production, so that it will be possible by careful selec- 

 tion to obtain an excellent material. 



I have now for more than two years used the same disks ; and 

 when their action was not energetic, I soon restored it by rub- 

 bing them with magnesia and petroleum *. When the weather 

 was very damp this proved very efficient ; if exposed to 

 moderate heat, it was sufficient to rub the disks with the hands 

 to render the machine active as soon as they were put in the 

 proper position. These precautions are easily practised, be- 

 cause the construction is very simple and there is no chance of 

 breaking any part. Often, as I have observed, the alteration 

 of the ebonite is but superficial ; in this case the insulating pro- 

 perty is recovered when the surface has been again polished. 

 At all events this change will not readily ensue when the ma- 

 chine is kept, after use, out of direct daylight ; and it is also 

 very convenient in this case to place the disks on a horizontal 

 surface ; this will remove the curved superficies which appear, 

 when their diameter does not exceed 2 millims. 



Exciting the machine. — The machine with disks of ebonite 

 has the remarkable property that, although its action is as 

 energetic as that obtained with glass disks, it is never excited 

 when the inclined or diametral conductor is not present |. It 



* This liquid is a very excellent developer of the electric qualities of 

 ebonite. 



t This name is applied to the conductor which is fixed in the axis be- 

 fore the rotating disk, opposite to the paper armature, and inclined at an 

 angle of nearly 70° to the horizon. 



