Exp. 10. When sparks are passing between the con ductor and a brass bail inches distant, present a sharp point at double that distance, and the sparks will no long er appear.
The brilliancy of the electrical spark is proportional to the conducting power of the bodies between which it passes. Hence metals are used for the production of the electric spark. When an imperfect conductor, such as wood, is employed, the electric light appears in the form of faint red streams. The length of the spark depends upon the power of the electrical machine, and the state of the weather when the experiment is made. With a very powerful electrical machine sparks may sometimes be procured from 10 to 20 inches long, and these sparks always pass from the positive to the nega tive ball.* The electric spark always varies its character with the nature and density of the medium through which it is transmitted. The earliest experiment upon this subject was made by Dr Watson, on a large scale, and is one of the most beautiful which can be exhibited.
Eali. 1. Take a glass cylinder, three feet long and three inches in diameter, and fit it up at one end so as to be able to let clown a brass plate from the top, in order to approach another brass plate fixed at the bottom of the cylinder. Let the cylinder be now exhausted and insulated. When the upper plate is electrified, the elec tric matter will pass from one plate to another, at the greatest distance at which the plates can be pitied, and the lower plate will be electrified as if it had been con nected with the prime conductor. When the room is dark, the electric matter, which in the open air passes in small brushes or pencils of light about an inch or two long, will now be transmitted in vivid coruscations of a bright silver hue along the whole length of the tube. These coruscations do not divide as in the open air, but from a base apparently flat, they frequently (livid( them SC1VCS into smaller and smaller ramifications, like the coruscations of the 1101•11E11 lights. When the vacuum is very perfect, the electric matter will pass butts ern the plates in a continued stream of the saint size throughout its whole length.
E.rp. 2. Having placed the brass plates at the distance of 10 inches, make this vacuum a part of the circuit through which a jar is discharged, and at the instant of the discharge, a mass of brii;lit embodied fire will be seen to leap from one of the brass plates in the tube to the other. When the distance of the plates is greater
than 10 inches, the light begins to diverge and lose its force, and this divergency is nearly proportional to the distance of the plates.
Exp. 3. Form a Toricellian vacuum in the angular part of a long bent tube of glass filled with mercury and inverted, and place the legs of the bent tube in separate basons of mercury. if this vacuum is insulated, and one of the basons of mercury made to communicate with the conductor, while some non-electric substance touches the other, the electric light will pervade the vacuum in a continued arch of lambent flame, without the least di vergency. If one of the basons be connected with the machine, which should be insulated, the light will per vade the vacuum in a contrary direction.
These three experiments were made by Dr Watson.
Exp. S. Through the top of the receiver of an air pump, a foot high and half a foot broad, insert a pointed wire, so as to project two inches into the inside. Place the receiver on the plate of the air-pump, and connect the wire with the prime conductor of the electrifying machine. A brush of light will appear at the point of the wire ; but as soon as the exhaustion of the receiv er commences, this brush will enlarge itself, and vary its shape ; and when the rarefaction is considerable, the whole of the receiver will be filled with a beautiful blush of light, the colour of which changes with the strength of the electricity.
Exp. 9. Into the extremity of a glass tube SO inches, insert an iron wire projecting into the tube, and hating a ball on the outer end of it. Fill the tube with mercu ry, and at the open end place a drop of ether, and secure it by the finger, while the tube is inverted so as to form a Toricellian vacuum in the upper part The ether will rise to the top, and upon the removal of the finger, and the descent of the mercury, it will expand into vapour. If electricity is now transmitted through this vapour, it will become luminous, and assume various hues, accord ing to its strength. If a strong spark passes through some inches of the vapour, the light is generally of a blue colour.