150 EXPERIMENTS'. 



As the excitation which is produced by rubbing with the 

 hand on a tube or plate of glass, is not only very laborious, 

 but inadequate to the production of any material quantity of 

 electric fluid, machines have been constructed of various 

 forms for this purpose. The most common machine con- 

 sists of a glass cylinder, supported by two glass pillars, and 

 made to turn by a crank or handle. A rubber, or cushion, 

 of leather, spread with an amalgam of mercury and zinc or 

 tin is fastened to a spring, which proceeds from a socket ce- 

 mented on the top of another glass pillar. A piece of black 

 silk is fastened to the cushion and extended over the cylin- 

 der, nearly to the receiving points, to prevent the fluid from 

 flying off. A fourth glass pillar supports what is called the 

 prime conductor, which is made of hollov^ brass or tin plate, 

 and, at the end towards the cylinder, has a collection of 

 pointed wires, and at the other end, a single wire terminated 

 by a brass ball. A small chain is fastened to the cushion, 

 one end of which extends to the floor or table. It serves 

 to conduct the fluid in passing from the earth to supply the 

 machine. When the cylinder is turned swiftly, the electric 

 fluid passes from the rubber to the glass, and is thence con- 

 veyed to the points of the prime conductor, which is thus 

 positively electrified. While the electric fluid is collecting, 

 it produces a crackling noise, and in a darkened room the 

 flame will be seen spread on the surface of the cylinder. If 

 a cylinder be made of resin, the electricity is the reverse of 

 that wliich is produced by tlie smooth glass cylinder and 

 rubber of the usual machines ; for in this case the rubber 

 partakes of the positive, and the cylinder, and prime conduc- 

 tor, is electrified with the negative. This difference be- 

 tween the resin and glass has given rise to v/hat is called the 

 double current, or vitreous and resinous electricity ; but it is 

 generally supposed that the difference arises more from the 

 eflfect of the surfaces that act on each other, than from -any 

 peculiar qualities in the different bodies. 



Some of the experiments which may be made with an 

 electrical machine are necessary for illustrating the laws of 

 electricity, and others are merely entertaining. If the inside 

 of a glass tumbler be electrified by presenting it to a pointed 

 wire extending from the prime conductor, and then placed 

 over a few pith-balls laid upon a table, the balls will immedi- 

 ately begin to lexip up along the sides of the glass, and then 



