# Electrical resistance/corrosion/wear. Best combination of metals for application.

Hello. I manufacture a fishing alarm that is activated when 2 very small rods (each about ½ inch long and the diameter of a small paper clip) come into contact. The rods are perpendicular to each other and form an “X” shape. When the rods do touch to complete the circuit, the surface area of the physical contact is VERY small, being that the rods are round and only contact at the point at the center of the “X”. The amount of pressure to make the contact is also VERY small and can be accomplished with a feather. Because the contact point of the 2 perpendicular rods and pressure is so small I need to find the best combination of metals and plating that will allow the lowest possible electrical resistance, will stand up to light abrasion as the 2 rods as they rub against each other, and will also not corrode or oxidize easily. The circuit is powered by a 3 volt coin cell battery and runs at 5ma. Here are a few questions and any help would be greatly appreciated. Thanks in advance.

1. Does the base metal or plating metal determine the resistance of the rod? For example if rod is steel and plated with gold would the resistance of the rod be based on the plating material or the base material or a combination? If the answer is that the plating metal controls resistance would using a thicker layer of plating lower resistance even more? Would it matter if I added a layer of nickel between the steel and gold or multiple layers like nickel/gold/nickel/gold?

2. When a metal is rubbed against itself, are there other factors besides “hardness” that would determine how fast it wears? For example say I have 4 rods, 2 made of steel and 2 of copper and they are exactly the same size. If I put the 2 steel rods in contact with one another, and the 2 copper rods in contact with one another, and then spin the rods, with both sets having the exact same amount of pressure on each, will they both wear out evenly in the same amount of time or will the harder metal wear faster/slower? Are some metals better to withstand rubbing against itself?

3. I need the surface of my plated rods to be as smooth (less peaks and valleys) as possible because of the low physical pressure and small contact point. Are some base metals or plating metals better at allowing a smooth surface?

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Hi Vtinventor,

I'm from Italy, so as first, since I'm not a mothertongue, I hope to have correctly understood your question. Otherwise please accept my apologizes in advance.

About your doubths and even considering I'm not a physicist, I just wish to share with you my direct field experience for similar matters related to instrumentation.

About your questions I try to advice you as follow, even if I would appreciate the further contribution of more skilled people than me:

1) I believe that the resistance of the plated rod, will be due the inverse contribution of each metal used, as obvioously, the more conductive will be the preferred one by the current flow, but on the other hand it shall be considered the amount of available material for current flowing (available section), that obviously, in the case of more conductive plating material, will be as well as more will be the thickness of plating. You do not mention the entity of power you need to switch with your "X" rod contact, but considering your application (fishing alarm), I guess like a very limited amount of current should be, so I do not trust so much crytical your worries about the thickness of plating, and its rod material, as basically both are metals, and not significative Joule skin effect should affect your application.

2) Considering the same conditions of operation for different metals, I would think that the more hard would be the survivor, agains the less hard. Obviosly some long terms basis considerations should apply, but basically, considering your application context (you mentioned like a very small amount of pressure will actuate the "X" rod switch, I would not expect significative rod "wearing". Then if we also will take in account the limited amount of flowing current (see my previous item), I don't think that either, contact wearing or current sparks may affect your "X" switch. Probably, considering the very small dimensions of your rod, and considering the concept of "better to be safe than sorry", should be not so more expensive to increase the plating thickness, in order to increase the contact life.

3) At last, and considering your application from a single point of view, I can offer you my relatively limited experience I had with slip rings, where silver coin is very often used (at least for small signals in the mV order, against true power transmission, between rotating and stationary parts). In my knowledge, silver coin is an alloy of silver and copper, that may vary (under some brand name), from 10%Cu/90%Ag, up to 30%Cu/70Ag. As well know, Silver is one of lowest resistance metal. I've been told that the reason of use of such alloy (Ag/Cu) should be addressed to the fact that the surface oxidation of such alloy is a better conductive mean that silver itself. Then, you should consider that any plating should offer a very smooth surface and this should fit with your application. About "wearing" I can add that silver coin is used for manufactuting of several coin in several countries, and one of the reasons of its use, is due the fact that it is relatively resistant to contact wearing.

What about to place more than a single "X" rod switch in parallel, in order to increase sensitivity and contact reliability?

I hope the above will help. On the other hand, I would ask the community here to add, modify or correct me if anything not accurate.

With all the best
Steve

Hi Steve and thank you for the information. Every bit helps!