Page:The American Cyclopædia (1879) Volume VI.djvu/516

 seen to better advantage if the room is darkened. By employing a hollow metal globe five or six inches in diameter, held by a glass handle, longer sparks may be obtained, as a greater quantity of negative electricity will be retained upon it before this discharge takes place. When the distance is short the spark is usually straight; when greater, it becomes curved, with branches or forks; and when considerable, say a foot, in large machines, it takes a zigzag course like that of lightning. When the air is dry and the machine working well, a hissing sound is heard, and in the dark luminous brushes are seen to issue from projecting parts of the conductor, and it will be found that the escape of positive electricity gives larger brushes than negative. Brilliant sparks will often leap across the plate between the cushions and the conductor, when the fluid is not otherwise taken away. By passing the spark through rarefied air and other gases various pleasing and furious effects are obtained. An apparatus called the electric egg, fig. 11, is often employed for this purpose. It consists of a glass globe with a cap at either end, through which passes a metal rod with a knob upon the interior extremity. The upper rod is made to slide so that the distance between the two knobs can be regulated at pleasure. Through the lower there is an opening provided with a stopcock, which may be screwed to a stand or to the plate of an air pump. When the egg is filled with air the spark has the ordinary appearance, but when partially exhausted it passes more readily, generally presenting the form given in the figure, the negative knob being usually surrounded by a violet halo, while the positive one exhibits a purple or a deep crimson. As the air becomes more rarefied the resistance to the discharge diminishes, and the brilliancy of the spark decreases, unLeM a nearly perfect vacuum is obtained, when no discharge will take place, as has been shown by Gassiot. These experiments are advantageously performed with the Holtz machine FIG. 11. Spark in Rarefied Air, by connecting the two poles with the two knobs of the egg, or other apparatus. The color of the spark depends on the gaseous medium and on the material of the conductors. When the spark is strong a portion of the conductor appears to be volatilized, from the fact that examination with the spectroscope reveals bright lines denoting the presence of metallic vapor, which are characteristic. When the spark is weak the influence of the gaseous medium is readily observed, nitrogen producing a blue, hydrogen a red, and carbonic acid a green color. The old electricians devised many pieces of apparatus for making curious and pleasing experiments, such as tubes, globes, and plates of glass, having small disks of tin foil placed at regular distances, distributed in spiral and other forms, so that when a spark was passed through the system it would be multiplied and exhibit illuminated figures. A person standing upon an insulated stool, and connected with the prime conductor, will become electrically excited, and sparks may be taken from the face or hands, or through the clothing, the resistance offered by the latter often causing the discharge to be quite loud and attended with a smart sting. The hair of the insulated person, if not too long, will stand upon end, and if dry each hair will be separate. A set of radiating arms like the spokes of a wheel, having their points bent in one direction, balanced upon an upright pointed rod supported by FlQ 12 ._ Eevolving Points, the prime conductor, fig. 12, will revolve when the machine is in action; an effect caused by the repulsion between the electricity of the points and that of the adjacent air which has been charged by them. A point placed upon the conductor, as in fig. 13, will create a current of air which may be felt by the hand or made manifest by placing a candle before it. A phenomenon usually accompanying the passage of a stream of electricity from a point, or of the spark, is the conversion of a portion of atmospheric oxygen into its allotropic condition, ozone, which is easily perceived by the sense of smell. The conversion is supposed to be FIG. 18. Current shown by means of a Flame.