CHAPTER VII.

CURRENTS IN VACUO.

Notwithstanding it requires an intensely high potential to enable the current to jump an air gap of 1 inch, the same potential will produce a luminous discharge through exhausted glass tubes aggregating 8 feet or even more.

But the exhaustion can be carried so far that there is no apparent discharge; and, on the contrary, air at as high a pressure as 600 pounds per square inch will resist the passage of the spark over an extremely short space. If the tubes be filled with various gases and then partially exhausted, the length of tube through which the luminous discharge will pass varies with the gas, becoming shorter in the following order: Hydrogen, nitrogen, air, oxygen, and carbonic acid—the shortest.

Before detailing some of the more striking phenomena connected with high-tension discharges in vacuo, a description of a few forms of simple mercurial air pumps will be serviceable.

Fig. 42: If a glass tube, _F_, stopped at one end, 3 feet long or over, be filled with mercury and the open end immersed in a vessel of mercury, _T_, the column of metal in the tube will sink until it attains a height, _M_, of about 30 inches, varying according to the condition of the atmosphere.

The space between the mercury column and the top of the tube will be a fairly good vacuum. This fact was noted many years ago, and the gradual evolution of the mercurial air-pump based on this result can be followed in the articles on the mercurial air-pump by Silvanus P. Thompson, read before the Society of Arts, England, some years ago.

Geissler, the first manufacturer of the "Geissler" or vacuum tube for electrical research, seeing the inconvenience of the above-described operation and the meagre results obtained, invented the pump called by his name (Fig. 43).

_F E_ is a stout glass tube some 3 feet long, having a bulb, _B_, at its upper extremity, and a rubber tube, _S_, attached to the curved end. A reservoir of mercury, _R_, connects with this rubber tube, and a special glass tap is fixed in the upper end of the glass tube at _E_, beyond which tap being the point of attachment for the object to be exhausted. The operation is as follows: On turning the tap a part of the way it allows a passage between the tube _F E_ and the atmosphere. The reservoir _R_ is then raised until the mercury flows into the bulb and up the tube to the tap. The tap is then turned a fraction, and the communication with the air is shut off and opened between the object to be exhausted and the tube _F E_. The reservoir is then lowered and the mercury falls, drawing down the air from the object into the tube. The tap is then turned as in the first place, and the reservoir _R_ raised, when the air drawn into the tube is forced out by the rising column of metal. This operation being repeated many times, withdraws nearly all the air from the object—in fact, makes a fairly good vacuum. This pump has been much modified from the simple form described.

The form of pump most used in the United States lamp factories is based on the application of the piston-like action of a quantity of mercury dropping down a tube. This is known as the Sprengel pump, after the inventor.

Fig. 44: _F_ is a stout glass tube about 40 inches long by one-twelfth of an inch internal diameter, carrying the reservoir funnel _R_ at the top, a piece of soft rubber tubing, _S_, nipped by a pinch-cock being interposed to admit of the regulation of the mercurial drops. The lower end of this "fall tube," as it is called, is immersed in mercury contained in a suitable vessel, _V_, a branch tube being blown or cemented into the fall tube to admit of the connection of the object to be exhausted at _E_. _S_ is another piece of rubber tubing with a pinch-cock regulation. The point _H_ is the normal barometric height of the mercury—about 30 inches. On attaching a bulb, for example, at _E_, and regulating the pinch-cock at the top of the fall tube _F_, a succession of drops of mercury falls down the tube, each drop acting as a piston to drive the air before it, sucking the same from the bulb, and forcing it down through the tube and vessel out into the atmosphere.

On its first being set into operation, the cushions of air between the drops silence their fall; but as a higher degree of rarefaction occurs, the air cushions become insufficient, and the drops fall with a sharp click on the top of the barometric column.

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.

Fig. 45 shows a solution tube to be filled with solution of sulphate of quinine, etc.

Fig. 46 shows three exhausted tubes arranged in series.

_A_ is of uranium glass, and glows dark green; _B_ of English glass, showing a blue hue, and _C_ of soft German glass, glowing with a bright apple-green tint.

Crystals of nitrate of calcium, nitrate of silver, benzoic acid, tungstate of calcium, lithia benzoate, sodium salicylate, zinc sulphide, and acetate of zinc fluoresce.

Fig. 47 is a highly exhausted tube, having at its lowest part a few pieces of ruby. When the secondary current is turned on at _P_ and _N_ the rubies shine with a brilliant rich red, as if they were glowing hot.

Fig. 48 shows the tube to exhibit the effect resulting from focussing the electric rays on a piece of iridio-platinum at _B_.

The cup _A_ forms the negative pole; the metal disk _C_, the positive.

On increasing the intensity of the spark, the metal at _B_ glows with extreme brilliancy, and melts if the intensity be carried too far.