April 4, 2013 in Uncategorized

By Steve Kirby

Most of you know that those of us who own an early Jag (or most any other British car up through the late 60’s) have vehicles that originally came fitted with Positive Ground electrics, while most of rest of the world, then and now, had Negative Ground equipment.  Much has been said and written about how to convert positive ground cars to negative ground, if for no other reason than to allow one to plug their iPhone charger into the lighter socket (don’t try this with a positive ground lighter or Mr. Lucas will let all the smoke out of the wires).  But little is ever said about WHY these cars were Positive Ground in the first place.

By way of background, positive and negative ground simply refers to the direction that electrons flow through the wiring when something electrical is turned on.  In a conventional negative ground car, the “hot wires” are hooked to the “+” side of the battery, from where “juice” flows to the device to be powered, let’s say a headlight.  The “juice” then returns to the “-“ side of the battery, frequently through the metal parts of the vehicle chassis.  This enables a device to be powered by a single hot wire without the need to have a separate “return” wire.  (Author’s note:  Ok you electrical engineers out there please don’t bother to write me to correct this explanation of current flow, I know the electrons actually go the other way, but that’s too confusing for this article).

In a positive ground situation, the roles, and the cables, are reversed.  The “hot” wires are connected the negative terminal of the battery, and the positive terminal is connected to the chassis.  But why so?  Who came up with this oddball orientation and why?

Over the years I have asked many smart electrical guys, from auto techs to electronics engineers, and no one has had a rational reason for how positive ground evolved, until I read Rod Shears excellent article entitled “Improving XK Charging” in this month’s XK Gazette.  It seems that up until the invention of PVC (plastic) insulated wiring in the 60’s, all wiring was covered with cloth based insulation, usually varnished to make it more durable.  This cloth based insulation was not perfect, especially when it absorbed water as it was wont to do in Jolly (and foggy) Old England.  Under these conditions, a small amount of electricity tended to “leak” through the insulation wherever wiring came in close proximity to a ground (like running through the firewall).  Today we refer to this phenomenon as (lack of) dielectric strength (the ability of something to contain electricity, i.e. to insulate).

Engineers of the day found that where this “leakage” occurred, corrosion would form.  We’ve all seen the effects of corrosion around electrical terminals, batteries and the like.  However, it was noted that when the ground was Positive, the corrosion effect was greatly reduced.  Now you know.  Faced with imperfect insulation and a wet climate, English auto makers opted for the least corrosive electrical orientation.

Want more?  Ok, so after the invention of plastic wire insulation, why did Positive ground go away?  Electronics.  Sometime around this era the transistor was developed and all kinds of neat things were invented….including electronic ignition and transistor radios which were cool to have in cars.  The electronics industry standardized on what is known as the NPN transistor (negative-positive-negative) which only works on Negative ground electrical systems!  Hence, positive ground cars became a thing of the past.

And now you know, probably more than you ever wanted to.  Thanks to Rod Shears and the XK Gazette.  Rod’s article also includes a lot of valuable info on converting XK Jags to negative ground, going from generator (DC current) to alternator (AC current) and related subjects.  Order issue #185 at