PHYSICAL PROPERTIES OF THE SPARK.

593

596. Mechanical and Physical Properties of the Spark.-The electric spark produces a violent commotion in the medium in which it occurs. This is easily shown by means of Kinnersley's thermometer (Fig. 371), which consists of two glass tubes of unequal diameters, the smaller being open at the top, while the larger is completely closed, with the exception of a side passage, by which it communicates with the smaller. The caps which close the ends of the large tube are traversed by rods terminating in knobs, and the upper one can be raised and lowered to vary the distance between the knobs. Both tubes are filled, to a height a little below the lower knob, with a very mobile liquid such as alcohol. When the spark passes between the knobs, the liquid is projected with great violence, and may rise

to a height of several yards if the spark is very strong. The same property of the spark is exhibited in the experiment of the electric mortar, which is sufficiently explained by the figure (Fig. 372).

The spark may be obtained in the interior of a non-conducting liquid, which it agitates in a similar manner. If the liquid is contained in a closed vessel, this is often broken. The spark can also traverse thin non-conducting plates, producing in this case perforation of the plates; but the experiment usually requires very powerful discharges, such as can only be obtained by means of apparatus which will be described in Chapter xlvii.

The luminosity of the electric spark is probably due to the very high temperature which is produced in the particles traversed by the

discharge. Coal-gas is easily inflamed, by a person standing on a stool with glass legs holding one hand on the conductor of the machine, and giving sparks from a finger of the other hand to the burner from which the gas is issuing. Kinnersley regarded elevation of temperature as the cause of the movement of the liquid in his apparatus; hence the name which it bears.

Heating may also occur in the case of conductors. This is shown by the influence of the metal upon the colour of the spark, and it may be more directly proved by arranging a conductor in communication

with the earth, and connected by an exceedingly fine metallic wire with another conductor. When the latter is presented to a very powerful electrical machine, so that a strong spark passes, the fine wire is sometimes heated to redness.

597. Chemical Properties of the Spark.-The electric spark is able to produce very important chemical effects. When it occurs in an explosive mixture of two parts of hydrogen with one of oxygen, it causes these gases instantly to combine. This experiment is usually shown by means of Volta's pistol (Fig. 373), which is a metallic vessel, containing the mixture, and closed by a cork. Through one side

CHEMICAL EFFECTS.

595 passes an insulated metallic rod with a knob at each end, that at the inner end being at a short distance from the opposite side of the vessel, so that, if a spark is given to the exterior knob, a spark also passes in the interior, and inflames the mixture. This effect is accompanied by a violent detonation, and the cork is projected to a distance.

The electric spark often produces a reverse effect-that is to say, the decomposition of a compound body; but the action in this case is gradual, and a great number of sparks must be passed before the full effect is obtained. Thus, if a succession of sparks be passed in

the interior of a mass of ammonia, contained in a vessel inverted over mercury, the volume of the gas is observed to undergo a gradual increase, until at length, if kept at constant pressure, the volume is exactly doubled. It then consists of a mechanical mixture of nitrogen and hydrogen, the constituents of ammonia.

Composition and decomposition are often both produced at once. Thus, if a spark is passed in a mixture of carburetted hydrogen and a certain proportion of oxygen, the former gas is decomposed, its hydrogen combining with a portion of the oxygen to form water, and its carbon combining with another portion to form carbonic acid.

598. Wind from Points.-If a metallic rod terminating in a point be attached to the conductor of the electrical machine, electricity escapes in large quantity from the point, which, accordingly, when viewed in the dark, is seen to be crowned with a tuft of light. A layer of air in front of the point is electrified by contact, and then repelled, to make way for other portions of air, which are in their turn repelled. A continuous current of air is thus kept up, which is quite perceptible to the hand, and produces a very visible effect on the flame of a taper (Fig. 374).

The electric whirl (Fig. 375) consists of a set of metallic arms, radiating horizontally from a common centre about which they can turn freely, and bent, all in the same direction, at the ends, which are pointed. When the central support is mounted on the conductor of the machine, the arms revolve in a direction opposite to that in which their ends point. This effect is due to the mutual repulsion between the pointed ends and the electrified air which flows off from them.

Fig. 375.-Electric Whirl.

It is instructive to remark that if, by a special arrangement, the rotating part be inclosed in a well-insulating glass case, the rotation soon ceases, because, in these circumstances, the inclosed air quickly attains a state of permanent electrification.

Fig. 376.-Electric Bucket.

the equality of action issuing stream.

599. Electric Watering-pot.-Let a vessel containing a liquid, and furnished with very fine discharge tubes, be suspended from the conductor of the machine. When the vessel is not electrified, the liquid comes out drop by drop; but when the machine is turned, it issues in continuous fine streams. It has, however, been observed that the quantity discharged in a given time is sensibly the same in both cases. This must be owing to and reaction between different parts of the

CHAPTER XLVI.

ELECTRICAL POTENTIAL, AND LINES OF ELECTRICAL FORCE.

600. The object of the present chapter is to give a brief outline of the methods by which mathematicians have succeeded in bringing numerous electrical problems within the range of accurate reasoning.

The fundamental conception in the mathematical theory of electricity is that of attraction and repulsion, acting according to the law of inverse squares; and the unit quantity of electricity is defined to be that quantity which would attract or repel an equal quantity at unit distance with unit force.

The influence which an electrified body exercises in the region around it can be specified by stating the force of attraction or repulsion which the body would exert upon a small charged body placed in various parts of this region or field (which is accordingly called a field of electrical force), the force being stated not only in magnitude but also in direction. In this sense we speak of the electrical force at a point, meaning the force which would be exerted upon a unit of electricity placed at the point; and in any such specification the unit of electricity is supposed not to disturb by its presence the previously existing distribution of electricity.

There can be electrical force at points in the air, or in the substance of any non-conductor, without disturbance of equilibrium; but electrical force in a conductor instantly produces a current of electricity in the direction of the force. At the surface of a conductor electrical force can exist, but it must always be normal to the surface; for if there were any tangential component, a current would be produced along the surface.

601. Definition of Difference of Potential.-We know, by the principle of the conservation of energy, that the work done upon a unit