The Electric Spark.

148.] When the tension in the space between the two electrodes is consid- erable all the way between them, as in the case of two balls whose distance is not very great compared with their radii, the discharge, when it occurs, usually takes the form of a spark, by which nearly the whole electrification is discharged at once.

In this case, when any part of the dielectric has given way, the part next to it in the direction of the electric force is put into a state of greater tension, so that it also gives way, and so the discharge proceeds right through the dielectric. We may compare this breaking down of the dielectric to what occurs when we make a little rent perpendicular to the edge of a piece of paper and then apply tension to the paper in the direction of the edge. The paper is torn through, the disruption beginning at the little rent, but diverging occasionally so as to take in weak places in the paper. The electric spark in the same way begins at the point where the electric tension first overcomes the ‘electric strength’ of the dielectric, and proceeds from that point, in an apparently irregular path, so as to take in other weak points, such as particles of dust floating in the air.

149.] The investigation of the phenomena of the luminous electric dis- charge has been greatly assisted by the use of the spectroscope. The light of the spark or other discharge is made to fall on the slit of the collimator of the spectroscope, and after being analysed by the prisms is observed through the telescope. The light as thus analysed is found to consist of a great number of bright lines and bands forming what is called the spectrum of the light. By comparing light from different sources it is found that these bright lines may be divided into groups, each group being due to the presence of a particular substance in the medium through which the discharge takes place.

SPECTRUM OF THE ELECTRIC SPARK

By using the method introduced by Mr. Lockyer of forming an image of the spark upon the slit by means of a lens, we may obtain at one view a comparison of the constituents of the medium which are rendered luminous by the dielectric discharge at the different points of its path. Close to either electrode the lines are principally due to the metal or metals of which that electrode consists. At greater distances these lines become fainter, thinner, and less numerous, but the spectrum belonging to the gas through which the discharge takes place remains visible.

Some of the lines due to the metals appear longer than others, shewing that they can be formed in regions of the spark where others are no longer visible, owing either to a deficiency in the amount of the metallic vapour or to a want of vigour in the electric disturbance. It thus appears that the electric discharge separates an appreciable amount of matter even from the hardest metals and carries the particles through the air to a distance of several millimetres from the surface of the metal. It also appears by a comparison of sparks from different electrodes and through different gases that no part of the light is emitted by any substance common to all the different cases, but that every line is due to one or other of the chemical substances present.

It follows from this that neither the electric fluid, if there be such a sub- stance, nor any ethereal medium such as is supposed to pervade all ordinary matter, is rendered luminous during the discharge, for if it were so its spec- trum would be visible in all discharges.

Steady Currents.

150*.] In the case of the current between two insulated conductors at dif- ferent potentials the operation is soon brought to an end by the equalization of the potentials of the two bodies, and the current is therefore essentially a Transient current.

But there are methods by which the difference of potential of the conduc- tors may be maintained constant, in which case the current will continue to flow with uniform strength as a Steady Current.DANIELL’S BATTERY.

The Voltaic Battery.

The most convenient method of producing a steady current is by means of the Voltaic Battery.

For the sake of distinctness we shall describe Daniell’s Constant Battery:— A solution of sulphate of zinc is placed in a cell of porous earthenware, and this cell is placed in a vessel containing a saturated solution of sulphate of copper. A piece of zinc is dipped into the sulphate of zinc, and a piece of copper is dipped into the sulphate of copper. Wires are soldered to the zinc and to the copper above the surface of the liquid. This combination is called a cell or element of Daniell’s battery. See Art. 193.* 151*.] If the cell is insulated by being placed on a non-conducting stand, and if the wire connected with the copper is put in contact with an insulated conductor A, and the wire connected with the zinc is put in contact with B, another insulated conductor of the same metal as A, then it may be shewn by means of a delicate electrometer that the potential of A exceeds that of B by a certain quantity. This difference of potentials is called the Electromotive Force of Daniell’s Cell.

If A and B are now disconnected from the cell and put in communica- tion by means of a wire, a transient current passes through the wire from A to B, and the potentials of A and B become equal. A and B may then be charged again by the cell, and the process repeated as long as the cell will work. But if A and B be connected by means of the wire C, and at the same time connected with the battery as before, then the cell will maintain a con- stant current through C, and also a constant difference of potentials between A and B. This difference will not, as we shall see, be equal to the whole electromotive force of the cell, for part of this force is spent in maintaining the current through the cell itself.

A number of cells placed in series so that the zinc of the first cell is con- nected by metal with the copper of the second, and so on, is called a Voltaic Battery. The electromotive force of such a battery is the sum of the electro- motive forces of the cells of which it is composed. If the battery is insulated it may be charged with electricity as a whole, but the potential of the cop- per end will always exceed that of the zinc end by the electromotive force of the battery, whatever the absolute value of either of these potentials may be.

The cells of the battery may be of very various construction, containing dif- ferent chemical substances and different metals, provided they are such that chemical action does not go on when no current passes. 152*.] Let us now consider a voltaic battery with its ends insulated from each other. The copper end will be positively or vitreously electrified, and the zinc end will be negatively or resinously electrified. Let the two ends of the battery be now connected by means of a wire. An electric current will commence, and will in a very short time attain a constant value. It is then said to be a Steady Current. Magnetic Action of the Current.