RECENT PROGRESS IN PHYSICS. 



369 



Fif. 5 represents a macliine, about one-ninth its actual size, witli a 

 15-incli plate, giving sparks seven to nine inches in length. The 

 axis i of the plate, as well as the supports h gf and I, are of glass. 

 The rods h carry the axis of the plate, / bears the rubber, g the con- 

 ductor, and I the discharger. 



Fiir. 5. 



The prime conductor a, the spheres b and c, are all of sheet brass. 

 To diminish, as far as possible, the loss of electricity by the support 

 g, the conductor a has the form advantageously used in the great 

 Harlem machine by Van Marum, as shown in section in fig. 6. 



The conductor carries two wooden rings d, between which the plate 

 revolves. These rings are of polished wood, and are provided, on the 

 sides opposite the glass plate, with strips of tin foil^ from which the 

 collecting points project. These strips are continued to the conductor 

 a, to which they carry the collected electricity. 



From the bulb h projects a wooden rod about one inch in diameter, 

 and rather more than a foot long ; to this is attached a wooden ring 

 about two feet in diameter, whose section is equal to that of the rod. 

 Both the rod and the ring are covered with tin foil. 



The discharger N is connected with the conductor of the rubber by 

 a metallic cord m, enveloped in silk ribbon. 



To obtain negative electricity, we have only to detach the cord m 

 from the conductor of the rubber, and put the conductor a in con- 

 nexion with the ground. 



. The arrangement for holding the rubber is shown detached in 

 fig. Y. Upon the glass rod / stands the fork-shaped piece of wood 

 24 s 



