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Bill Coburn (
Posted on Thursday, 09 January, 2003 - 04:37:   Edit PostDelete PostView Post/Check IP

I have often been asked why the immediate post-war cars (pre S series) changed their polarity from negative to positive earth and had to confess I have no idea. It seemed to be a popular idea that maybe came out of experience with various engines during the war. Many British cars went positive earth post war and there are tomes written about the techniques and effects of changing the cars over, mainly to permit the installation of modern audio equipment. The Americans were quite ambivalent before the war with Westinghouse and Delco trying both systems. I can find no writings on the theoretical basis for the selection of either polarity.

Can anyone enlighten me on why Rolls-Royce made the temporary switch?
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Jim Bettison (
Posted on Thursday, 09 January, 2003 - 10:28:   Edit PostDelete PostView Post/Check IP

The information that I have in memory suggests that the selection of polarity by RR was at one stage influenced by information they had collected regarding corrosion in cooling system components. Around the time of introduction of the B series engines the problems of light metal decay began to surface in a serious way - accentuated, perhaps, by the problems of metal "purity" akin to the problems that also affected the steel used in bodies of the time. That's largely my surmise. I do know that RR did a lot of work on corrosion in multi-metal systems, largely prompted by the Merlin aero engine, and one of the earliest papers I have seen on the inhibition of corrosion in engines was by an RR engineer (but do you think I can lay my hands on it?). He gave the whole background to the selection of inhibited ethylene glycol for corrosion inhibition, and mentioned the "metal mix" of alloys used as being important. But I digress. I have also come across a passing suggestion in some accounts of RR engine development that negative earth was beneficial in countering electrolytic corrosion; did that lead to the changes you mention?
Apart from all that, I suspect that the configuration of car radios as to chassis polarity played a part, and increasingly so post-war as radios were increasingly fitted. In this regard I know that the preferences and practices of semiconductor manufacturers (particularly of power devices) played a part, and although it's no big deal to opt for either polarity upwards, the cost-shaving practices of equipment manufacturers was certainly influential in some degree.
I don't expect that this stroll around the field has done much to answer your question. But it does suggest an interesting line to follow in the RR archive at the RREC in Paulerspury. I reckon two packs of sandwiches and a thermos of coffee would be the opening provision list ...
Jim B.
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Bill Coburn (
Posted on Friday, 10 January, 2003 - 08:00:   Edit PostDelete PostView Post/Check IP

Thanks for that. The only theoretical writings I have found on the subject have been the odd Factory Bulletins going to what appears to an old novice, inordinate trouble to get bondings right between various bits of the chassis including the engine. And then there is often talk of the sacrificial bits on the engine, the favourite being the thermostat cover which is of course preferable to one of the covers on the back of the cylinder head or block! I have just been re-reading the article by Norman Geeson on corrosion in postwar blocks in the RREC bulletin (Issue 253)and wonder whether in 50 years all that will be left of the vee eight engine will be a bottle of salts sitting on the mantlepiece! Now starting to think that I am corroding despite consuming ample doses of preservative, at great cost.

As you say it is WHY they (presumably the Board) opted for the change that interests me and no doubt Paulesbury holds the key!
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Jim Bettison (
Posted on Friday, 10 January, 2003 - 10:11:   Edit PostDelete PostView Post/Check IP

Yes, they did go to an enormous amount of trouble to fix whe electrolytic action problem. Certainly, if one goes poking around the various elements of the engine (particularly, the radiator, water pump, head and generator) with a millivoltmeter various different potentials can be measured. Without having reversed the polarity of our MkVI, I have no info to comment on the effect that might have.
I spent several days with Norman Geeson in October. He is a mine of information (is that possible?) and presently is following through on the evolution of the B series engine, particularly as regardsthe cylinder block - its design and development and changes introduced during the production; but that's another story. The article to which you refer is the first half; a second part has yet to appear. I have a copy of the complete article; I think that he has edited the second part.
More later.
Jim B.
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chrisg (
Posted on Sunday, 12 January, 2003 - 12:50:   Edit PostDelete PostView Post/Check IP

An article by Tony Ward on changing a vehicle's polarity may be of interest, as it addresses some of the reasons for the change. It will be appearing in the next "London & Derby" but a copy has been posted in the Online Library. You may view it at
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Bill Coburn (
Posted on Sunday, 12 January, 2003 - 13:38:   Edit PostDelete PostView Post/Check IP

Yes Chris I had read Tony's account among others. I am amused by the pejorative phrase 'it is perfectly simple' that was quoted out of an RREC paper. Are we to draw the conclusion that Rolls-Royce and its band of elves were not aware of these problems and assuming they were, why may I iterate, did they make the change? The answer I reckon lies in the Hunt House among technical submissions to the Board. Someone will find them someday.
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Richard Treacy (
Posted on Sunday, 12 January, 2003 - 22:10:   Edit PostDelete PostView Post/Check IP

Negative chassis ? Positive chassis ? Who cares ?

Yes, the change-over is relatively simple, and I considered it for about five minutes once for my R-Type. However, I didn't want to disable or modify my beautiful and fully-functional HMV radio, although I suspect it works on either polarity as its power supply is from a mechanically-driven AC transformer. To fit the FM radio & CD player I simply insulated the mounts and isolated the antenna earth. No problem, and you can tuck the new electronic wizard away and not butcher the original radio or woodwork. Any talk that a CD cannot be fitted to these cars is bunk. Also, I didn't want the possibility of needing to fool around with blower fans, heater fans, instruments and so on. I am nor sure off hand, but some of these devices could work backwards if unaltered with a negative chassis. Most new players have a floating chassis anyhow, as do my reverse parking sensors and cruise control. I am more than a bit sceptical about this corrosion stuff, which has more to do with careful earthing of the block, head, radiator and chassis. Besides, the polarity only determines the source and destination of the bits that move. R-R always paid extra attention to earthing. Try counting the number of earthing braids on an R-Type. Phew. As a novelty, on the V8s there are even 8 (oil leak inducing !) earthing tabs between the rocker covers and the cylinder heads (part no. UE34258). Hmnn.

I think that the positive chassis arrangement was only made because it was the UK motor industry norm in the early post-war period, and saved the RAC a little pain. Later, when the UK industry realised a motor industry actually existed outside the UK, all those manufacturers progressively adopted negative earthing in accordance with evolving world norms.

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chrisg (
Posted on Monday, 13 January, 2003 - 21:55:   Edit PostDelete PostView Post/Check IP

You certainly have a point there, Richard. I love the old (valve?) radio in my Mk VI, even though it takes so long to warm up that by the time any sound comes out I've forgotten I turned it on. It's invariably quite a shock when it does!

Not that I listen to radio much when driving - I prefer to listen to the car's own music. So I'm not in the market for "hi-fi" anyway.

When I first owned Bess, however, we took a trip to Adelaide from Sydney. Anticipating the long, straight roads across western NSW I brought along a "ghettoblaster" to play tapes to help keep me awake (I'm not good on such roads).

Not wanting to buy batteries for the player I simply wired up a 12-volt cigarette lighter socket to the loom with negative earth and let it float inside the glovebox. The player sat on the back seat with a power cable to the socket. An easy solution, largely invisible and readily removed if desired.

Of course, it would be just as easy to make a polarity-reversing cigarette lighter plug for a few dollars; then I could have connected it to the socket in the passenger reading lamps and not had cable running across the car.
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Martin Cutler (
Posted on Wednesday, 15 January, 2003 - 21:41:   Edit PostDelete PostView Post/Check IP

Many years ago I fitted a modern radio to our MG Magnette. The radio was isolated, and the aerial isolated as well. All went well until the car was painted, and the apprentice who put the battery back in put it in negative earth. Didn't hurt the car, but blew the smithereens out of the radio! Haven't replaced it yet, so am tuneless! I have been threatening to get a radio in the MK VI, maybe soon.
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Bill Coburn (
Posted on Friday, 17 January, 2003 - 09:03:   Edit PostDelete PostView Post/Check IP

These accounts make me realise how fortunate I am. Having had a 1950 Armstrong Siddeley Whitley for some years I always hankered for a radio but again was hit with the positive earthing problem. Seems it was a post war design fashion in the fifties.I stand correcting, but Siddeley's carried positive earthing through until their demise in 1960. A friend whom at the time I really did not know well gave me a brand new Zenith Radio dual polarity and AM/FM tuning. What more could I want. The original was those quaint units that ran from a box under the dashboard which was tuned from a little console screwed under the facia and tuned with flexible cables. The noises produced were reminiscent of the Goon Show but I suppose it was better than nothing. I was once told that when Hythe Rd was in full flight and you took your car in for a 'general overhaul' - usually at about 100K miles one of the things discarded without hesitation was the factory fitted radio which was replaced with the latest instrument. The last example of this I remember was a replacement made in the Vice Regal Phantom VI some years ago. My Bentley S2 has an original radio which shall be preserved,and new equipment installed albeit with an eye to 'returnablity' for the sake of originality.
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David Gore (
Posted on Friday, 17 January, 2003 - 09:18:   Edit PostDelete PostView Post/Check IP

Martin, There are a number of radio preservation societies dedicated to preservation of valve radios who may be able to point you in the direction of a technician who can rebuild your radio - the only part that should have failed would be the vibrator transformer which was always a cantankerous beast at the best of times. My parents FJ Holden was retrofitted with a Phillips dual polarity valve radio around 1956 and this survived in subsequent cars until I traded their XL Falcon in 1971. The only problem was having to replace the vibrator unit every 2/3 years as the contacts failed due to the continuous use of the radio by yours truly.
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Bob (
Posted on Sunday, 19 January, 2003 - 02:44:   Edit PostDelete PostView Post/Check IP

The polarity with modern antifreezes and rust proofing is now of little concern in that respect.

If you want to fit modern radios etc then neg earth is best.

Clocks are sometimes polarity consious.
Clocks are easier to convert than radios.

If you are into originality then stay with the factory polarity.

In any case it is worth fitting a sticker near the battery to inform anybody working on the car of the correct polarity.

These are available from Lucas Service.

I too had an MG Magnette with a HMV valver and the thing was a nightmare for using up battery power.

Modern equipment is far far better.

Which is in keeping with RR policy of fiiting the best equipment.

Any work should be done so that it can be reversed easily.