Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/95

57.] The Electric Spark.

57.] When the tension in the space between two conductors is considerable all the way between them, as in the case of two balls whose distance is not 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 parts on either side of it in the direction of the electric force are put into a state of greater tension so that they also give way, and so the discharge proceeds right through the dielectric, just as when a little rent is made in the edge of a piece of paper a tension applied to the paper in the direction of the edge causes the paper to be torn through, beginning at the rent, but diverging occasionally where there are weak places in the paper. The electric spark in the same way begins at the point where the electric tension first overcomes the insulation 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 air.

On the Electric Force required to produce a Spark in Air.

In the experiments of Sir W. Thomson the electromotive force required to produce a spark across strata of air of various thicknesses was measured by means of an electrometer.

The sparks were made to pass between two surfaces, one of which was plane, and the other only sufficiently convex to make the sparks occur always at the same place.

The difference of potential required to cause a spark to pass was found to increase with the distance, but in a less rapid ratio, so that the electric force at any point between the surfaces, which is the quotient of the difference of potential divided by the distance, can be raised to a greater value without a discharge when the stratum of air is thin.

When the stratum of air is very thin, say .00254 of a centimetre, the resultant force required to produce a spark was found to be 527.7, in terms of centimètres and grammes. This corresponds to an electric tension of 11.29 grammes weight per square centimètre.

When the distance between the surfaces is about a millimètre the electric force is about 130, and the electric tension .68 grammes weight per square centimètre. It is probable that the value for