300 SEC. 10. ELECTRICITY. 



The Cecchi electrical or dielectrical machine is composed of two discs 

 with parallel axes. The upper disc is of india-rubber, and is supported by 

 an axis of glass ; the one below is of glass on an axis of metal. The axis of 

 the glass disc has on one side a large pulley, and the axis of the other disc a 

 small pulley, and by means of a continuous cord, not crossed, there is trans- 

 mitted to the caoutchouc disc a rotatory motion eight or ten times faster than 

 that of the glass-disc ; both the discs turn in the same direction. The discs 

 are partially placed one above another, and are very close but without touching. 

 The upper part of the caoutchouc disc passes between tAvo arms furnished 

 with metallic points, and connected with a large sphere of brass insulated at 

 the extremity of a long glass rod. To this sphere is attached the hook of a 

 condenser or else a Leyden jar formed by a barometer-tube with very thick 

 walls. The lower part of the same disc passes before a comb of metallic 

 points, called a T-comb, which communicates with the external armature of 

 the condenser, and with two friction cushions of the glass disc, and then with 

 the ground, and also with an exciter formed by a tube of brass with a ball at 

 the end. When the discs are revolving, the large sphere becomes charged 

 with negative electricity. This machine with discs of 80 centimetres diameter 

 has given sparks of the length of 42 centimetres free in the air. 



1242. Holtz's Machine, with four plates. M. Ruhmkorff. 



1242a. New form of Holtz's Machine. 



Francis Pizzorno, Bologna, Italy. 



This machine has the property of working well whatever may be the hy- 

 grometric condition of the air. since the two glass plates are placed in 

 a crystal case hermetically closed. The air of the case is maintained con- 

 tinually dry by means of drying substances. 



The conductors issue from the case and unite at the two Leyden jars which 

 appear on the front of the figure. 



1242b. Fixed Disc for a Holtz's Machine. 



Augustus Righi^ Professor of Natural Philosophy, Bologna. 



The greatest possible difference of potential between the conductors of a 

 Holtz's machine depends on the difference of the potentials of the paper sur- 

 faces carried by the fixed disc. But this latter difference is limited by the 

 discharges which continually occur along the fixed disc. An ebonite plate is 

 joined perpendicularly on the fixed disc, separating it into two parts, so that 

 the discharges must follow the two faces of the plate. The potentials of the 

 paper surfaces are increased, and the sparks between the conductors become 

 longer. 



This machine possesses four rows of points, namely, the two rows of points 

 of the conductors, and the two rows of points obliquely communicating. 



1242c. Large Electrical Machine, with double ebonite 

 plates and Waiter's ring, formerly the property of Lord Lindsay. 



Dr. Stone. 



1268. Replenishes Designed by Sir W. Thomson for re- 

 storing electricity to the Leyden jar of his quadrant electrometer. 



Elliott Brothers. 



A small charge being given to the Leyden jar, the replenisher increases or 

 decreases the difference of potentials between the two coatings of the jar by a 

 constant per-centage per half turn. 



