Length of Spark

—The width of the spark gap or the sparking distance between the terminals of an induction or spark coil, being the distance through air that the disruptive discharge can take place, varying as the difference of potential and the pre ure of the air. The length of spark increases with the electric pre ure. It diminishes with an increase of air pre ure, hence, the high voltage required for ignition of internal combustion engines (10,000 to 30,000 volts) due to the compre ion of the fuel mixture. The length of spark varies for different gases, for instance, it is nearly twice as long in hydrogen as in air at the same density. The voltage required to produce a given length of spark depends on the shape of the electrodes and not on the kind of metal used. Pointed electrodes produce the longest spark with a given voltage. ` Faraday, using two spheres of different sizes as electrodes, found the spark length greater when the smaller sphere was positive than when it was negative. A perfect vacuum is a perfect insulator; no spark will cro it.