Disruptive Discharge

133.] If the electromotive force acting at any point of a dielectric is gradu- ally increased, a limit is at length reached at which there is a sudden electrical discharge through the dielectric, generally accompanied with light and sound. The dielectric, if solid, is often pierced, cracked, or broken, and portions of it are often dispersed in the form of vapour. This phenomenon appears to be analogous to the rupture of a solid body when exposed to a continually increasing stress. This analogy is so complete that we may make use of the same terms in describing the behaviour of media under the action of elec- tromotive force as we apply to bodies under the action of stress.

Thus electromotive force and electric displacement correspond to ordinary force and ordinary displacement; the electromotive force which produces disruptive dis- charge corresponds to the breaking stress. Conduction, or the transmission of electricity, corresponds to permanent bending.

Thus if we consider the twisting of a wire on the one hand, and the trans- mission of electricity through a body on the other, the moment of the couple which twists the wire will correspond to the electromotive force acting on the body, and the angle through which the wire is twisted will correspond to the electric displacement. If the wire, when the force is removed, returns to its former shape and becomes completely untwisted it is said to be elastic. Such a wire corresponds to a dielectric which acts as a perfect insulator with re- spect to the electromotive force employed. If the twisting couple is increased up to a certain limit the wire gives way and is broken. This corresponds to disruptive discharge, and the ultimate strength of the wire corresponds to the greatest electromotive force which the dielectric can support, which we may call its electric strength.

MECHANICAL ANALOGIES

If before rupture takes place the wire yields so that it will no longer com- pletely untwist itself when the force is removed it is said to be plastic. It corresponds to a dielectric which conducts electricity to a certain extent. If no such permanent twist can be given to the wire by a force which is not sufficient to break it, the wire is called brittle. In like manner we may speak of those dielectrics such as air, which will not transmit electricity except by the disruptive discharge, as electrically brittle.

134.] Many wires after being kept twisted for some time and then set free immediately untwist themselves, but through a smaller angle than they were twisted. In the course of time, however, they go on untwisting themselves, but very slowly, the process sometimes going on for days or weeks. In like man- ner many dielectrics such as the glass of a Leyden jar or the gutta percha of a submarine cable, after being subjected for some time to electromotive force and then placed in a closed circuit give an instantaneous discharge which is less than the original charge. After this discharge, however, they are capable of giving residual discharges which become more and more feeble, and if the circuit is kept closed a quantity of electricity will slowly ooze out through the circuit, the current becoming feebler and feebler as the charge is more nearly exhausted.

Mechanical Illustration of the Properties of a Dielectric.

135*.] Five tubes of equal sectional area A, B, C, D and P are arranged in circuit as in the figure. A, B, C and D are vertical and equal, and P is horizontal. The lower halves of A, B, C, D are filled with mercury, their upper halves and the horizontal tube P are filled with water. A tube with a stopcock Q connects the lower part of A and B with that of C and D, and a piston P is made to slide in the horizontal tube. Let us begin by supposing that the level of the mercury in the four tubes is the same, and that it is indicated by A0 , B0 , C0 , D0 , that the piston is atMECHANICAL ANALOGIES. 126 P0 , and that the stopcock Q is shut. Now let the piston be moved from P0 to P1 , a distance a. Then, since the sec- tions of all the tubes are equal, the level of the mercury in A and C will rise a distance a, or to A1 and C1 , and the mer- cury in B and D will sink an equal dis- tance a, or to B1 and D1 . The difference of pressure on the two sides of the piston will be represented by 4a.

This arrangement may serve to repre- sent the state of a dielectric acted on by an electromotive force 4a. The excess of water in the tube D may be taken to represent a positive charge of electricity on one side of the dielectric, and the excess of mercury in the tube A may represent the negative charge on the other side. The excess of pressure in the tube P on the side of the piston next D Fig. 29.

will then represent the excess of poten- tial on the positive side of the dielectric. If the piston is free to move it will move back to P0 and be in equilibrium there. This represents the complete discharge of the dielectric. During the discharge there is reversed motion of the liquids throughout the whole tube, and this represents that change of electric displacement which we have supposed to take place in a dielectric. I have supposed every part of the system of tubes filled with incompressible liquids, in order to represent the property of all electric displacement that there is no real accumulation of electricity at any place. Let us now consider the effect of opening the stopcock Q while the piston P is at P1 .

The level of A1 and D1 will remain unchanged, but that of B and C willRESIDUAL DISCHARGE.

become the same, and will coincide with B0 and C0 . The opening of the stopcock Q corresponds to the existence of a part of the dielectric which has a slight conducting power, but which does not extend through the whole dielectric so as to form an open channel. The charges on the opposite sides of the dielectric remain insulated, but their difference of potential diminishes. In fact, the difference of pressure on the two sides of the piston sinks from 4a to 2a during the passage of the fluid through Q. If we now shut the stopcock Q and allow the piston P to move freely, it will come to equilibrium at a point P2 , and the discharge will be apparently only half of the charge.

The level of the mercury in A and B will be 12 a above its original level, and the level in the tubes C and D will be 12 a below its original level. This is indicated by the levels A2 , B2 , C2 , D2 . If the piston is now fixed and the stopcock opened, mercury will flow from B to C till the level in the two tubes is again at B0 and C0 . There will then be a difference of pressure = a on the two sides of the piston P. If the stopcock is then closed and the piston P left free to move, it will again come to equilibrium at a point P3 , half way between P2 and P0 . This corresponds to the residual charge which is observed when a charged dielectric is first discharged and then left to itself. It gradually recovers part of its charge, and if this is again discharged a third charge is formed, the successive charges diminishing in quantity. In the case of the illustrative experiment each charge is half of the preceding, and the discharges, which are 12 , 14 , &c. of the original charge, form a series whose sum is equal to the original charge. If, instead of opening and closing the stopcock, we had allowed it to remain nearly, but not quite, closed during the whole experiment, we should have had a case resembling that of the electrification of a dielectric which is a perfect insulator and yet exhibits the phenomenon called ‘electric absorption.’ To represent the case in which there is true conduction through the dielec- tric we must either make the piston leaky, or we must establish a communi- cation between the top of the tube A and the top of the tube D.ELECTRIC STRENGTH OF AIR.

In this way we may construct a mechanical illustration of the properties of a dielectric of any kind, in which the two electricities are represented by two real fluids, and the electric potential is represented by fluid pressure. Charge and discharge are represented by the motion of the piston P, and electromotive force by the resultant force on the piston. 136.] The electric strength of a dielectric medium depends on the nature of the medium and its density and temperature. Thus the electromotive force required to produce a disruptive discharge is greater in glass or ebonite than in air.

The electric strength of air or any other gas may be tested by causing sparks to pass through a portion of the gas between two balls of metal. If the exper- iment is conducted in a glass vessel from which the air may be exhausted by an air pump, it is found that the electromotive force necessary to produce the discharge diminishes, while the pressure is reduced from that of the atmo- sphere to that of about 3 millimetres of mercury. If the supply of electricity is kept up at a constant rate, the sparks become smaller and more frequent, till at last there appears to be a continuous flow. If, however, the exhaustion be carried further, the electric strength again increases, till in the most perfect vacuum hitherto made the electromotive force required to produce a spark be- tween electrodes ·6 centimetres apart is so great that the discharge does not take place between the electrodes, but passes round the outside of the vessel through a distance of 20 centimetres of air at the ordinary pressure. It would therefore seem as if a perfect vacuum would present an almost insuperable resistance to the passage of electricity. A small quantity of gas, however, introduced into the empty space renders it incapable of withstanding even a small electromotive force. This diminution of the electric strength, however, does not go on when the density of the gas is still further increased, but for pressures of a centimetre and upwards the electric strength increases as the density increases.

137.] The electric strength of air diminishes rapidly as the temperature rises. The heated air which rises from a flame conducts electricity freely. The best way of discharging the electrification of the surface of a solid dielectric is to pass the electrified body over a flame. In most experiments with heated air the air is in motion. It is therefore desirable that experiments should beELECTRIC STRENGTH OF AIR.

made on the conductivity of air at various temperatures, contained in a closed vessel and free from currents. 138.] In order to test the insulating properties of air and other gases I made the following experiment:— Fig. 30.

A tube half an inch in diameter, CD, is supported on an insulated stand c. A rod AB, a quarter of an inch in diameter, is supported by the insulating stand a so that about 6 inches of the rod is within the tube with a cylindrical shell of air about an eighth of an inch thick between it and the inside of the tube. The tube is connected with one electrode of a battery of 50 Leclanché cells, the other electrode being connected to earth. The rod is connected to one electrode of Thomson’s quadrant electrometer, the other electrode being connected to earth. A tube, F, which is fixed so as not to touch the tube CD, is used for sending a current of hot air or steam through the tube CD. The part of the tube CD which contains the rod AB is surrounded by a wider tube E of thick brass which may be heated by a gas furnace so as to keep the inner tube and rod hot without exposing them to the current of the products of combustion of the burner.

The sensitiveness of this apparatus was shewn by the effect of communi- cating a small charge to the tube E. The electrometer was immediately de- flected on account of induction between the tube and the rod AB. The rod AB was then discharged to earth so that the electrometer indicated zero, the tubeCONDUCTIVITY OF GASES.

remaining at a higher potential. If any conduction were now to take place through the air between the tube and the rod it would be indicated by the electrometer. No conduction however could be observed even after the lapse of a quarter of an hour, and when hot air and steam were blown through the tube. At the end of the experiment the tube was discharged to earth, when a negative deflection of the electrometer was observed, shewing that the tube had remained charged during the whole experiment.