George7941 is correct - the center electrode is hotter than the side electrode. Both center and side electrodes are exposed to the heat/flame of combustion. But heat from the center electrode travels a longer distance to the cooler cylinder head than the side electrode, which is just a short distance to the threads of the spark plug, which are already in tight contact with the cylinder head.
Wikipedia also says this:
The central electrode is usually the one designed to eject the electrons (the cathode) because it is the hottest (normally) part of the plug; it is easier to emit electrons from a hot surface, because of the same physical laws that increase emissions of vapor from hot surfaces (see thermionic emission). In addition, electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, i.e. from a sharp point or edge rather than a flat surface (see corona discharge).
Wikipedia also says this:
The central electrode is usually the one designed to eject the electrons (the cathode) because it is the hottest (normally) part of the plug; it is easier to emit electrons from a hot surface, because of the same physical laws that increase emissions of vapor from hot surfaces (see thermionic emission). In addition, electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, i.e. from a sharp point or edge rather than a flat surface (see corona discharge).
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