One great disadvantage in this form of pump is the tendency to fracture of the glass tube that is manifested by the concussion of the drops of mercury at the barometric height. However, this has to a certain extent been obviated in later forms of this useful and efficient pump.
For many electrical experiments, the simple exhaust tube (Fig. 42) mentioned at the beginning of the article will be found very satisfactory. The top end need not necessarily be sealed off with glass, a cork having a wire, W, run through for connection being driven in, and a coat of paraffin or one of the cements mentioned in a later chapter be laid on.
The second electrical connection is made by a wire dipping in the tumbler of mercury.
Discharges in Vacuo.
In a simple glass tube having two wires carrying balls inserted through its ends, from which the air has been partially exhausted, the study of the changes shown by the passage of the spark is extremely interesting. Before the commencement of exhaustion no luminous effect can be discerned; at a low degree of exhaustion a luminosity appears between the ends of the wires, the negative pole being surrounded by a violet glow and a larger pear-shaped red discharge from the positive. An interval near the negative electrode is in darkness, widening as the exhaustion progresses. When the degree of exhaustion is very high, a series of arches concentric with the positive ball appear and become broader and more distinct as the rarefaction progresses. The arches or bands are called striæ, and are most distinct when the tube is made in the form of a narrow cylinder, with a bulb at each end. Carbonic acid gas vacua give the best results. If the finger be placed on the bulb at either end a luminous spot appears, and by using a very rapid contact breaker in the primary circuit, the luminous discharges become highly sensitive, being diverted from their regular path on the approach of the hand, a magnet, or a grounded wire. An extended treatment of these phenomena would be out of place here, but can be found in nearly all comprehensive works on electricity.
If an incandescent-lamp bulb be held in the hand and one end be brought near to a terminal of the coil, a beautiful bluish light appears.[2] The carbon filament, if long, and not held by its loop, becomes electrified and oscillates, often giving out a clear, high, bell-like sound as it strikes the glass. Particles of carbon deposited on the glass during the burning of the lamp, shown in daylight as a blackening deposit, generally show little sparks, like stars scattered over the inside of the globe.
[2] This depends on the degree of exhaustion.
A vacuum tube will phosphoresce if held in the hand near a secondary terminal, or even if laid on the table near the coil, and will light quite brilliantly if one end be held against a terminal. This latter method, however, is generally inconvenient, as a certain amount of physical pain ensues from the discharge into the skin.
Different gases in the tubes give characteristic colors. In carbonic acid gas the whitish green hue prevails; in hydrogen, white and red; in nitrogen, orange yellow. The characteristic spectra are given by the gases in the tubes, and can be readily examined in the spectroscope. But there is sometimes a slight variation in these colors, dependent upon changes in the current.
In many Geissler tubes, a portion of the bulbs is made of uranium glass. On the passage of the spark in the tube this glass glows with a magnificent emerald green hue. Other tubes are constructed with an outside enveloping glass tube fitted with a corked orifice into which can be poured different solutions.