+ve. or -ve. earth?

I knew I had a reference to this somewhere. From "The Modern Motor Engineer" by Arthur W. Judge 1952:

The Positive Earthing System

It has been mentioned elsewhere that the earlier method of connecting the negative terminal of the battery to the chassis frame, or " earth," has more recently been superseded 'by that of earthing the positive terminal, and it can be stated that this method has definite electrical and other advantages over the other one.

When the positive pole of the battery is earthed instead of the negative one, the central electrodes of the sparking plugs then become negative to the metal shells of the plugs, so that any corrosion or burning away of the plugs is confined to the relatively large area of the metal shell and not to the nickel-alloy central electrode, the plugs will last much longer on this account. Another advantage is that the metal tongue on the H.T. rotor arm of coil-ignition systems becomes negative to the metal contacts of the H.T. distributor casing to which the sparking-plug cables are attached, so that any burning away of the metal is confined to the relatively large-area metal contacts and not to the small-area metal tongue of the rotor arm; in negatively earthed systems the latter becomes burnt away and the jump-spark gap increases progressively.

When the positive pole of the battery is earthed it is also necessary to earth the positive pole connections of the dynamo and to reverse the ordinary ammeter connections of the negative-earth system. It is also possible to reverse the polarity of a dynamo designed for use with the latter system, using the battery for this purpose.

Previously wrote:

I knew I had a reference to this somewhere. From "The Modern Motor Engineer" by Arthur W. Judge 1952:

The Positive Earthing System.....

Not quite right as it happens. Yes the burning of the spark plugs is affected by the polarity of the spark but this is reversed (or not) by the coils connections. Hence all modern cars with negative earth do NOT suffer badly burn shorter life plugs. This phenomenon is shown when you use a double ended coil. One spark is positive while the other is negative. Inspecting the spark plugs after some mileage will show plug burning different between the two. One will have the traditional burnt centre electrode-correct, the other will have a burnt side(earth) electrode (poor). Try looking in a 2CV service manual some time....reverse the plugs when servicing-they burn different. (guess who uses a twin outlet coil?)

Dynamos-yes you can re-polarize for 'the other polarity' by feeding the LIVE side of your battery to the field coil, as simple as that. Certainly no need to run the engine. Also I do not know of any positive terminal on a dynamo. Again read the above with interest but not total belief please.

Al Osborn.

I give up. I have just spent 15 minutes replying and this useless system has lost what I wrote!

Alan

Hi Alan,

What I ALWAYS do is before hitting the Post button is I select all and do a Ctrl-C to copy what I have just written. Then if the post vanishes I can do a Ctrl-V to paste it in again.

Started doing this after the same frustratingexperienceas yours.

Tony