# Is silver wire a better choice to wind coils?



## Tesseract (Sep 27, 2008)

unclematt said:


> I saw this about silver wire replacing copper due to the increasing cost of copper making silver appear more attractive to manufacturers. Everything I can find, what little there is, makes the case for silver wire over copper....


Well, there's the economic factor. Silver is approx. 40% more conductive than copper but the spot price is $9/ troy ounce while copper is $1.70/lb. Thus, silver costs about, oh, 77x more than copper. Silver is also less strong than copper so less tension can be applied to it when winding motor coils, transformers, etc.

I'd say you get the best bang for the buck when silver plating copper, especially when high frequencies and/or high currents are involved. Even at 60Hz the resistance of a 1/8" x 1" bus bar is 5% higher than it is at DC because of the skin effect (the tendency for alternating currents to vacate the center of a conductor as frequency goes up).

This effect, btw, is the main reason for using Litz wire (composed of many fine strands of individually insulated wire bundled together).


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## unclematt (May 11, 2008)

Thanks for your post. If I am going to build custom electric motors for some ev's, I expect to have them for some time, if not forever. If the car wore out they were in, I would transplant them into a newer car, as I would the expensive batteries if they were still within operating lifetime. I personally wouldn't mind the added cost under those conditions.

Plus it helps if you ever have to run over any vampires or werewolves...
(j/k)

I wonder how thick of a coating of silver you need on copper wire to be effective at conducting the skin effects you mention. Then you would probably need enamel over the silver to prevent tarnishing.


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## Tesseract (Sep 27, 2008)

Okay - that makes more sense... Frankly, I had a hard time getting over the 77x difference in cost between silver and copper as quoted on the COMEX and LME.

Good news for you: silver tarnish conducts extremely well - there's no need to worry about it at all, even at the kiloamp level.

As for how thick a coating of silver you need to make it worthwhile, well, that depends on how much AC ripple current flows through the motor and at what frequency. In turn, the ripple current depends on the motor inductance and PWM duty cycle. In other words, it's not really practical to estimate. Silver plating is probably worthwhile at the power levels involved; silver wire, though? Eh, not so much.


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## unclematt (May 11, 2008)

Even so, I see more and more people building BLDC motors with silver wire on the RC forums...

Sure, EV levels of power are much higher, but I am still going to be looking at silver when build time rolls around, and I may even try to find a supplier of silver lintz wire.


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## Tesseract (Sep 27, 2008)

unclematt said:


> Even so, I see more and more people building BLDC motors with silver wire on the RC forums...
> 
> Sure, EV levels of power are much higher, but I am still going to be looking at silver when build time rolls around, and I may even try to find a supplier of silver lintz wire.


Not trying to discourage you at all - silver *is* a better conductor than copper so losses *will* be lower. It is not always the case that Litz is a better choice, though, because the frequency and amp level have to be high enough to justify its usually 50% larger diameter for a given DC current capacity.

Some reputable wire suppliers (I have personally used these folks): 

http://www.newenglandwire.com/products.asp

http://www.mwswire.com/plated1.htm


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## major (Apr 4, 2008)

Tesseract said:


> Well, there's the economic factor. Silver is approx. 40% more conductive than copper but the spot price is $9/ troy ounce while copper is $1.70/lb.


Hi Tesser,

My chart shows resistivity of silver at 9.9 and copper at 10.37 (annealed) and copper at 10.7 (hard-drawn). Units for both are Ohm*circular mills per foot at 20 degrees C. A far cry from 40%.

Regards,

major


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## unclematt (May 11, 2008)

major said:


> Hi Tesser,
> 
> My chart shows resistivity of silver at 9.9 and copper at 10.37 (annealed) and copper at 10.7 (hard-drawn). Units for both are Ohm*circular mills per foot at 20 degrees C. A far cry from 40%.
> 
> ...


Resistivity is inversely proportional to conductivity. Electrical resistivity (ρ) is also defined as the inverse of the conductivity σ (sigma):

ρ = 1/σ


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## Tesseract (Sep 27, 2008)

major said:


> Hi Tesser,
> 
> My chart shows resistivity of silver at 9.9 and copper at 10.37 (annealed) and copper at 10.7 (hard-drawn). Units for both are Ohm*circular mills per foot at 20 degrees C. A far cry from 40%.
> 
> ...


Don't know where you got your information from, maj, but here's just one reference I pulled at random from the web:

http://www.reade.com/Particle_Briefings/elec_res.html

Cu is 2.65 x 10^-8 and Ag 1.59 x 10^-8 ohms per meter. That would make silver exactly 40% more conductive than Copper.

If silver was only a few percent more conductive than copper why in the world would anyone bother plating copper bus bars, microwave cavitys, etc., with it when it costs some 77x more than copper?

Your numbers don't pass the common sense test.


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## major (Apr 4, 2008)

Tesseract said:


> Don't know where you got your information from, maj, but here's just one reference I pulled at random from the web:
> 
> http://www.reade.com/Particle_Briefings/elec_res.html
> 
> Cu is 2.65 x 10^-8 and Ag 1.59 x 10^-8 ohms per meter.


Hi Tes,

I think that web site is in error. Several others I just checked put it at 1.72 x 10^-8. I originally posted numbers from a copy of a page from a text book I've had nailed to my desk for about 10 years. Don't remember which book it came from. Also, it might be that the web site was giving a range. It says something like "1.673e2.65e-8", which is confusing. The resistivity of copper will vary with the specific alloy. But electrical grade copper should be close to 1.72 to 1.78.

Regards,

major


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## Tesseract (Sep 27, 2008)

I stand somewhat corrected. The difference in conductivity between _Aluminum_ and Copper is 40%... The most common alloys of copper used for wire are, indeed, only a few percent less conductive than silver while the less common alloys, such as beryllium-copper (used for springy contacts) are more similar to aluminum.

So, unclematt, it appears you might get a few percent improvement in DC conductivity if you wind a brushless motor with silver wire rather than copper. That's not particularly compelling to me, given the cost differential, but whether it is to you or not is a different story.


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## unclematt (May 11, 2008)

The link I posted also suggested that there are other reasons to use silver - and I quote:

"Coils of silver enamel wire produce a higher degree of electromagnetic flux creating more energy generating greater torque as compared to copper coils of the same weight."

and

"With almost same prices, imported silver enamel wire has an advantage over copper wire which is also its most important feature — interestingly only one third of silver enamel wire is enough to substitute copper wire. So this wire has not only reduced the cost of production but also the weight of electrical goods to a considerable extent."

Can you guys confirm this, or are they incorrect?

This may be off topic, but I always wondered what kind of coil performance you could get if you wound a SWNT (singe wall carbon nanotube) in a coild configuration and then passed electricity through it. Carbon nanotubes are ballistic conductors, and can conduct huge amounts of electricity for their size. I haven't been able to find out an answer to this question: Does a conductive carbon nanotube create a magnetic field the same as a metal wire? If you know the answer and have a reference, I would love for you to post.


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## major (Apr 4, 2008)

unclematt said:


> The link I posted also suggested that there are other reasons to use silver - and I quote:
> 
> "Coils of silver enamel wire produce a higher degree of electromagnetic flux creating more energy generating greater torque as compared to copper coils of the same weight."
> 
> ...


Hi uncle,

I'd say the guy that wrote that is full of crap.



> Does a conductive carbon nanotube create a magnetic field the same as a metal wire?


It is not the conductor which creates the magnetic field. It is the current, or flow of electric charge. Actually, a conductor is not even needed if you have a flow of charge, like an electron beam.

Regards,

major


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## Tesseract (Sep 27, 2008)

For once, major and I are in full agreement...


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## major (Apr 4, 2008)

Tesseract said:


> For once, major and I are in full agreement...


Hey Tesseract,

Sounds like the first time. Imagine that.

The guy that wrote that stupid article must own a sliver mine.

Have a nice day

major


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## koolkev (Jan 15, 2010)

The cost may be 77X but if the cost of copper is small compaired to the motor and the windings last forever and the power and weight savings are good than it may be worth it.

How many feet of copper wire with what type of insulation and what gage is needed to wind a particular motor, say 20 hp? 

It I can go 10 miles (10%?) more per given charge what would $3000 in wire be worth over a 10 year period?

I just was looking on line at a 20 hp motor that weighed in at 380lbs. I don't know why but it would be nice to cut that way down.


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## major (Apr 4, 2008)

koolkev said:


> The cost may be 77X but if the cost of copper is small compaired to the motor and the windings last forever and the power and weight savings are good than it may be worth it.
> 
> How many feet of copper wire with what type of insulation and what gage is needed to wind a particular motor, say 20 hp?


Hi kool,

How many pounds of wire is the correct question. Let's say 20 lbs of copper in the motor. And if you went to silver, for the same resistance, you could use 19 lbs. At today's spot prices, that would be an extra $5900 to save that one pound.

This is just the raw material cost. What do you think would be the cost to get the mill to pull silver in the correct dimensions and then coat it with the proper insulation? And to do copper, they usually have a 1000 pound minimum order.

You might as well buy the silver and put it in a safety deposit box hoping the price increase will do better than other investments. Putting it in the motor is a no-brainer. 

Regards,

major


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## koolkev (Jan 15, 2010)

The question is not how many pounds because the wire is not sold by the pound but by the foot. Then you could compare the actual costs. It certainly doesn't make sense to have a special wire pulled for one project, but if a wire exists already made the price may not be that high. 

The cost might be much higher than you estimate but I am asking for underlying data to make an informed estimate. 

The sites above have different wire and all of it is sold by the foot. Silver plated copper is much lower in resistance than plain copper. It may not be 77X the price. A quarter has significant silver on it and it doesn't cost 77X what a penny does. 

Is the amount of wire determined by the number of turns of a given diameter with a given gauge of wire?


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## koolkev (Jan 15, 2010)

The other question is what type of efficiency gain do you get with the 1 lb gain in weight. Certainly 1 lb is not a good return on $5000. But if you get other gains because of lower resistance, heat dissipation, horsepower gain. $5000 may not be a bad investment. Especially if the part you invest in will have a very long life. How many times can you rebuild an electric motor?


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## major (Apr 4, 2008)

koolkev said:


> The question is not how many pounds because the wire is not sold by the pound but by the foot.


When was the last time you bought custom size magnet wire? I think it does boil down to pounds. And if memory serves me correctly, a required 1000 lb minimum buy.



> Is the amount of wire determined by the number of turns of a given diameter with a given gauge of wire?


Sure, for a particular size coil. Realize that almost all the DC motors used by this DIY crowd use rectangular copper ribbon in armatures and series fields. The few that have shunt fields will use round wire, but that boils down to a handful of guys.


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## major (Apr 4, 2008)

koolkev said:


> The other question is what type of efficiency gain do you get with the 1 lb gain in weight. Certainly 1 lb is not a good return on $5000. But if you get other gains because of lower resistance, heat dissipation, horsepower gain. $5000 may not be a bad investment. Especially if the part you invest in will have a very long life. How many times can you rebuild an electric motor?


If you keep the mass of silver equal to the mass of the copper, then you will reduce the resistance approximately 5%. Resistive losses account for maybe about 2% of input at rated load. So that equates to a 0.001 (0.1%) increase in motor efficiency or put another way, an improvement in losses of about 1%. So for $5000 (material alone) you make a 88% efficient motor 88.1%.


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## yarross (Jan 7, 2009)

Tesseract said:


> ohms per meter.


Should be Ohm x m.



major said:


> So for $5000 (material alone) you make a 88% efficient motor 88.1%.


It makes more sense to use copper cage in induction motor instead of aluminium one. This provides about 2% eff gain and costs a lot less.
All this silver hype is an extension to audiovoodoo.


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## Tesseract (Sep 27, 2008)

yarross said:


> Should be Ohm x m.


Oh boy - this is the kind of thing that only an engineer can argue about 

You are correct that the specification is Ω · m but AFAIK you pronounce the phrase as "ohms per meter". No one I know says "ohms times meters". 

In defense of my position, I submit:

http://books.google.com/books?id=IH...8Q6AEwBQ#v=onepage&q="ohms per meter"&f=false




yarross said:


> It makes more sense to use copper cage in induction motor instead of aluminium one. This provides about 2% eff gain and costs a lot less.
> All this silver hype is an extension to audiovoodoo.


I agree _times_ two 

However, I *think* the main reasons Al is so popular in this application are it's low density and that it's easy to cast directly into the rotor slots. You can't do that with either Ag or Cu and, of course, both metals are much denser than Al so they would substantially increase the rotational inertia.

But, as usual, major may come along and shoot me down on this


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## major (Apr 4, 2008)

Tesseract said:


> However, I *think* the main reasons Al is so popular in this application are it's low density and that it's easy to cast directly into the rotor slots. You can't do that with either Ag or Cu and, of course, both metals are much denser than Al so they would substantially increase the rotational inertia.


I like the idea of copper cages. A lot of makers use them, but insert drawn bars and weld the shorting rings. Casting aluminum has been a big cost reduction with just a small reduction in efficiency. Recently a lot of work has been done to develop copper casting methods with some good success. I think several makers adopting the method for premium motors. I haven't heard of any progress with casting silver into rotors


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## yarross (Jan 7, 2009)

Tesseract said:


> However, I *think* the main reasons Al is so popular in this application are it's low density and that it's easy to cast directly into the rotor slots. You can't do that with either Ag or Cu


Casting process for Cu presents some challenges - higher copper melting point threatens lamination insulation, and there're problems with high porosity of resulting castings. I've read that pressurized bottom-pour process is the most promising.
There's a dedicated site for die-cast copper cage IMs:
http://www.copper-motor-rotor.org/







Tesseract said:


> and, of course, both metals are much denser than Al so they would substantially increase the rotational inertia.


I don't think that the increased rotor inertia is an issue for EV traction drives.


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## gor (Nov 25, 2009)

unclematt said:


> ...
> This may be off topic, but I always wondered what kind of coil performance you could get if you wound a SWNT (singe wall carbon nanotube) in a coild configuration and then passed electricity through it. Carbon nanotubes are ballistic conductors, and can conduct huge amounts of electricity for their size. I haven't been able to find out an answer to this question: Does a conductive carbon nanotube create a magnetic field the same as a metal wire? If you know the answer and have a reference, I would love for you to post.


 really interesting - 4 examle, what it could be...?


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## coulombKid (Jan 10, 2009)

History of the project to build the bomb has mention of the use of silver wire. The US government released hundreds of tons of .9999% silver to be drawn into wire for the Manhattan Project. With the good old US-of-A using the lions share of all generated electricity to purify weapons grade uranium the savings really added up. It also freed up the domestic market for copper that was being used over there. While in use on classified projects, the treasury department never really lost control of the silver stockpile. They never even missed it at Fort Knox much the the chagrin of the Japanese. I have steel pennies made during the war years to free up domestic copper for the war effort.


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## aeroscott (Jan 5, 2008)

1% motor eff is 10% less heat , motor builders stuff as much conductor as will fit in the slots . making the motor a little heaver then a copper one with the same size wire . I thought silver was 10% better on size / size . local hot spots silver has it .


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## coulombKid (Jan 10, 2009)

To really do a cost/benefit study one would need to consider things like change of total motor weight, propulsive efficiency of the entire vehicle, and life cycle cost reduction of the battery pack. My gut feeling is that silver is a winner. Using high grade VFD grade insulation and winding support would offset weaker mechanical strength of the wire. The insulators have come a long way in the last century. So has high resistance high silicone steel laminations. In heat transfer classes in E-school we had to calculate optimum insulation thickness for maximum heat transfer. Given the high dia-electric strength insulation available bear in mind thinner is not always better when it comes to heat transfer. A motor with silver windings needs the right type of insulation at the right thickness. Because the current flux density in the stator would be higher, one would need less wire and steel to produce the same Kw rated motor. In the rotor of an AC induction motor silver may or may not be easier to cast than aluminum. It should be much easier to cast that copper and less prone to air bubble inclusions. The high speed/high reliability production tricks to produce silver bar rotors have not been discovered yet so far as I know. When a set of usable Thunder Skys are setting us back 10 Gs I'd think the cost of silver would be a minor consideration. The motors here at work use 200 amps at 4160 volts 3-phase AC. Our annual power usage is so great that we should already have silver windings and silver bar rotors.


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## malysaad (Jan 23, 2010)

Any answer yet?





gor said:


> really interesting - 4 examle, what it could be...?


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## major (Apr 4, 2008)

malysaad said:


> Any answer yet?


See post #13.


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## gor (Nov 25, 2009)

found something interesting :

http://www.mutr.co.uk/product_info.php?products_id=1009623

Silverthread
conductive thread
[234-315] 







in stock £27.94
This is an economical, highly conductive thread for use in hand or *machine sewing*. This synthetic material feels and behaves like conventional cotton but is made up of over 100 strands each with a nano-coating of silver. It has an electrical resistance of just 4 Ω per 100mm – meaning that just a single thread can be used in place of wires in electrical circuits for LEDs etc. In experiments, we have even run electric motors using up to a metre of Silverthread as a substitute for normal metal wiring. This material can be *used in garments* and soft products as a woven-in conductor and to secure small circuit boards and wire components to fabrics. 

Sold per reel (approx. 50m)

p.s. Bing Search reccommends to check also "kitchen appliances" ... : ((///


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## aeroscott (Jan 5, 2008)

silver conductivity on size basis is 15.8 n ohms / m to coppers 16.78 n ohms / m divided = .94 or 6% better . as stated before motor build to space limits not weight limits .


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## aeroscott (Jan 5, 2008)

is 6% going to be 6% less heat . that's very big for the inner wires that are hardest to cool plus the better thermal conductivity for these inner wires


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## crap (Dec 5, 2009)

Tesseract said:


> Oh boy - this is the kind of thing that only an engineer can argue about
> 
> You are correct that the specification is Ω · m but AFAIK you pronounce the phrase as "ohms per meter". No one I know says "ohms times meters".
> 
> ...


Dunno if I wanna be that guy who marks words, but "ohms per meter" is only used for specific wires. For an example, a 1mm^2 wire would have about 0.0172 ohms per meter. However, when talking about a materials resistivity you usually just say "ohm meters".

Now, back to the ongoing discussion in the thread. Silver has about 7,5% lower resistivity, but it has about 17.6% higher density. Properly used, the weight of a motor rewound with silver would increase slightly. The advantage of using silver is the ability to decrease the electrical resistance for a specifik volume, since the volume of conductive material is often limited by the motor design. 

This is quite relevant in RC motors, since they have a very high part load and value power density highly. On a high load the conductive losses is the main enemy. Also, cooling is critical due to the high part load. The increased weight of the windings is small compared to the rest of the motor, and since the power can be increased it pays of.

In an electric car however, where the engine is running on a lower part load most of the time, conductive losses may not be the biggest enemy. Iron-losses, bearing losses, and all that is quite likely to be bigger.

My conclusion would be that silver windings in an electric car won't pay for itself in the form of increased range. It mainly pays of in the form of higher power-density. 

Of course, there could still be advantages to silver windings that I am not aware of...

Edit: I almost forgot, the resistivity of silver increases less with temperature. Still, that makes the biggest difference at high power outputs, once again the profit in part load efficiency is less.


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## aeroscott (Jan 5, 2008)

thermal conductivity is 429 w/m silver and 401 copper =106.9% , a big cooling aid .


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## crap (Dec 5, 2009)

aeroscott said:


> thermal conductivity is 429 w/m silver and 401 copper =106.9% , a big cooling aid .


Oh yeah, well said. Very useful when trying to increase the motors power. 

But once again not very important for part load use, in my opinion...


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## aeroscott (Jan 5, 2008)

a cooler running motor in all conditions will help but how much . stereo guys {some} say silver has advantages at similar frequencies that our controllers use .


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## gor (Nov 25, 2009)

2009 article about copper, silver and Nantennas (antennas)
http://www.thaindian.com/newsportal...ossibilities-in-communications_100167064.html

p.s. not related to copper vs silver:
nanotube wiring: 2009, Nasa, Airforce http://www.xconomy.com/boston/2008/...-to-make-carbon-nanotube-wiring-for-aircraft/


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## Haroldesic (Jun 27, 2014)

HELLO Unclematt,

My opinion is that, as a human society, we do not care about efficiency. We would rather wear pretty rings and make pretty money than to have truly free energy. If we would replace all copper with silver we could make much more efficient robust generators of all types, wind, water, solar, geothermal. 

This would alter the state of our society in a way that would eliminate silver as a material used for jewelry and pretty dishes. As a society we are not willing to give that stuff up. 

we would also have to shift the power structure from "those who control oil" to "those who control silver" This would be a costly change - why not just keep it the way it is ??? 

I also have been looking for a study that shows scientifically, pier reviewed, exactly, what the efficiency increase is for using silver. it sounds to me as if it is much more complicated and more efficient than the 6% TO 9% HIGHER conductivity of silver. 
when you consider the lower heat build up will mean a smaller cooling fan, potentially more coils in the same motor, less weight (for car motors). These factors work together putting out higher synergy. resulting in higher efficiency than a simple conductivity comparison. 
However; as long as these results are simply "guesswork" and not hard scientific studies we are doomed to do the experiments ourselves. spend tons of money on silver wire and compare our old hand crank generators to the new ones with silver wire. (I plan to do this but it will be a struggle because I am not equipped) 
thank you for your post I hope that if you find an answer you will let me know the results and keep in touch with me. Please reply to this post and email direct any reply to this post to oct1113 @ Hotmail.com 
I look forward to many replies
Harold


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