EARTHING THE POSITIVE

Southland Times, Issue 22999, 19 September 1936, Page 19

EARTHING THE POSITIVE

REVERSING AN OLD CUSTOM It has been the usual practice to earth the negative side of the electrical system, although there have been a few exceptions to this rule, says an article in The Autocar, London. Now however, there is to be a reversal of this, and it will gradually become the rule to earth the positive terminal of the battery. Indeed, on many of the 1937 models now in preparation it will be found that the positive side of the system is earthed. On the other hand, where existing models are retained it may be found that for a time the negative terminal continues to be earthed, for the change-over from negative earth to positive earth in some cases necessitates different lengths of cable, and existing stocks of cables have, in the interests of economy, to be used up. Naturally, the change-over is not being made without reason, and there are certain definite advantages to be obtained by earthing the positive battery terminal. As regards the lowtension side, there is less likelihood of corrosion of battery terminals, for it is the positive terminal which is most liable to attack when the negative terminal is earthed, since there will be a tendency, if moisture is present on the battery top, for the leakage current escaping from th positive terminal to earth, to cause corrosion at the positive terminal. When the positive terminal is definitely earthed, however, it will be immune from corrosion.

As regards the high-tension side, with the present arrangement of an earthed negative terminal, burning takes place on the rotating electrode of the distributor—that is, on the metal segment of the rotor arm. When the positive battery terminal is earthed, however, the rotating electrode will become negative instead of positive, and the burning, instead of being concentrated on the small central electrode, will be distributed over the four, six or more fixed electrodes in the distributor cover.

As far as the sparking plugs are concerned, burning at present takes place on the central electrode, which is positive, but when the positive terminal is earthed and the central electrode of the plugs is made negative, burning will take place at the outer electrode.

There is also another effect, for sparking plug voltage is dependent largly on the temperature of the negative electrode, the voltage falling as the electrode becomes hotter. As a general rule the central electrode of the plug is hotter than the earthed outer electrode, so that by making the central electrode negative, lower voltages will be necessary, and these naturally throw less stress on the ignition equipment There will, accordingly, be less liability to trouble due to the insulation of the high-tension cables deteriorating. It has also been found that when the central electrode is made negative there is less likelihood of misfiring at light loads.