Copper plating with copper sulphate. Copper plating at home

When it comes to electroplating, such technological operations as chromium plating and zinc plating of metal products immediately come to mind. But if you ask a question, what is electroplating, then not everyone will answer - it's checked. Although this term does not imply anything supernova.

Simply put, this is a technique for coating any material with the thinnest layer of metal, whether it be steel, aluminum, wood or plastic. We will figure out how to make copper plating of any sample at home.

general information

Copper plating is a somewhat more universal technique than the same zinc plating. For what purposes is it carried out?

Protection of samples from carburization before cutting them, as well as from corrosion.
Elimination of defects on the surfaces of parts, when other methods are unacceptable or difficult to implement. For example, if the base is characterized by a complex relief.
Decoration of products.
Creation of copies of samples from other materials.
Preparation of steel parts for silvering, gilding. In such cases, copper plating is only one of the stages of work on surface treatment material.
For creating solderable segments.

It is unlikely that the reader will be interested in such nuances as the classification of copper (refined, oxygen-free, general use), various options solutions used in copper plating, characteristics of materials and similar things. Further, only the simplest methods for applying Cu to any surface are considered, which are easy to organize at home, without any difficulties and financial costs.

Copper plating in electrolyte

This technique is only suitable for coating with a layer of Cu metal parts. In fact, the technology is not much different from the same galvanizing at home.

Training

The equipment you need is simple:

The bath (capacity) is glass. Its capacity is determined by the dimensions of the workpiece. Even liter jar or a glass - as options.

copper electrodes. Typically two are used. This allows you to better cover the workpiece with a layer on all sides and simplifies the process itself. In the course of work, you do not have to periodically change the position of the part relative to the electrode. What exactly to use depends on the specific situation - copper plates, pieces of thick wire. This is unprincipled.

Current source and connecting wires. Even a low-power power supply, 6 - 8 V, is enough. If the PSU does not have a built-in ammeter and smooth voltage adjustment is not provided, then you will have to use the appropriate device and the rheostat as separate elements electrical circuit. Approximate scheme, which is collected for copper plating of parts, is shown in the figure.

Electrolyte. You can use a purchased solution, although you have to look. If you cook it yourself, then for 100 ml of distilled water you will need sulfuric acid (3 ml) and copper sulfate (20 g) - not a shortage.

copper plating process

The part is cleaned of deposits. If necessary, it is pickled, immersed in special solutions to remove foreign particles. What exactly to use depends on the degree and type of contamination.
Sample degreasing. The easiest way is to dip in a solution of soda (hot), and then rinse with water to remove its residue.
The prepared composition is poured into the container and the electrodes are placed. The solution level is chosen so that it completely covers the workpiece.
Product immersion. It is tied up on a wire that connects to the "-" PSU. It is necessary to ensure that the workpiece does not touch the walls of the bath, its bottom and electrodes.

After switching on the voltage, the current gradually rises to the calculated value, and in this mode, processing is carried out for ⅓ hour (approximate time). If copper plating is carried out for the first time, then you should control this process. The fact that the part can be removed from the container is judged by the shade of its surface and the uniformity of the coating (the absence of untreated areas, shells, inclusions, and so on).

It remains only to wash off the remaining electrolyte from the sample and dry it. It turns out that this technology for implementation at home does not present any difficulty.

Copper plating without bath

In this way it is possible to apply metal coating for any materials. The bottom line is to “coat” (without direct contact) the workpiece with electrolyte with a special brush, the bristles of which are copper wires. The disadvantage of this technology is that it is unlikely that it will be possible to achieve high-quality copper plating of relief surfaces. At the very least, it will take a lot of time and effort to carefully process all the “crevices” and “potholes”.

Features of the preparatory stage

Tassel. At home, it is made from a stranded copper conductor. To remove the insulation and “fluff” one end of it is not a problem. To make it more convenient to work, you should think about what to make the brush handle from. She will have to drive along the surface of the sample, and given that the wires are flexible, such copper plating will be a test for the master. As an option - tie the "working part" to a pencil, a plastic case ballpoint pen. It's easy to guess.

Container. The part before copper plating is placed on any suitable dishes. For ease of use, it should not have high sides. The best option- plate. Plus, there is a container in which there will be an electrolyte. You will have to constantly lower the brush into it, so the choice is not difficult here either. A glass is also suitable if the sample is small and the solution is needed a little. Accordingly, all containers are pre-treated - washed, cleaned, boiled, degreased.

Schema assembly. Similar to the previous method. The brush acts as an anode, so it is to the “+” of the power supply, and the part to be coated is the cathode (to “-”).

copper plating process

To ensure the continuity of the electrical circuit, electrolyte is poured into the dishes so that its level exceeds the height of the part. A brush, which is also periodically dipped in the solution (for this it is poured into a separate container), must be driven over the sample. As a result, its surface is covered with a layer of copper. In fact, it is sprayed.

It is clear that such a process is more complicated in execution, since it is carried out in a “manual” mode. It is necessary to constantly ensure that there is a small gap between the brush and the base being processed. But this is not the main thing. Its invariance is one of the conditions for uniform coverage.

In what cases is it advisable to use this method of copper plating

If the sample material is not conductive.
For large parts. It is unlikely that it will be possible to choose a bath of the appropriate size at home, for example, for a chandelier.

How to determine the required parameters of the power supply? For current density during copper plating, the norm is 0.5 A / dm² of the sample, which is to be covered with a protective layer.

Exceeding the calculated value is fraught with the fact that copper will darken greatly, and besides, it will not hold firmly on the base.
With a complex configuration of the part, the presence of many protrusions, pointed segments, the current density is taken less, by about 2.5 times.

Copper oxidizes rather quickly. Before starting the processing of the product, the electrodes should be well cleaned.

The exposure time of the part in the solution is selected based on the thickness of the layer to be obtained during copper plating. The dependence is direct - the longer the treatment takes, the thicker the coating.

Restoration if needed appearance worn out elements of fittings (furniture or other), their copper plating is a good way out.

The author has more than once encountered the fact that people who are concerned about environmental problems immediately ask themselves the question - how to organize waste disposal at home? After all, the electrolyte is not eternal, and you definitely won’t be able to use it all your life. By the way, quite reasonable and more than fair remark.

There is a good solution - to collect the “burda” remaining after copper plating in a separate glass container. What for? Come in handy. This solution is excellent for wood processing. Your obedient servant, the reader, himself impregnated logs with it in front of the flooring in the country. Considering that it is not heated in winter, the operating conditions of the material are clear. When, 12 years later, it was necessary to shift the floorboards, it turned out that the logs were as good as new. There was not even the slightest hint of some kind of mold, traces of rot.

Since any of us has to deal with, if not construction, then certainly repair, it makes no sense to quietly, away from prying eyes, drain the used electrolyte somewhere. It's not businesslike.

The main task of copper plating at home or otherwise copper plating is to prepare the surface of the metal for its further processing. Various metals, and non-metals, can be subjected to such an operation, among which it is worth highlighting:

steel,
brass,
nickel and others.

Use of copper

Thanks to their numerous benefits this metal is widely used. Today, copper and its numerous alloys are widely used in industry. The metal is relevant for the aircraft industry, automotive industry, instrument making and other industries. Metal and products made from it are no less popular in the domestic sphere. Copper plating itself is one of the better ways coatings thin layer metal surface. At home, copper plating can be done in several ways.

Galvanic copper plating at home

For this you will need:

Water;
Hydrochloric acid in its purest form.

Galvanic copper plating at home

Solution preparation

We make a saturated solution of copper sulfate, after which it will be necessary to add 1/3 of this solution to hydrochloric acid. After preparing a solution of copper sulfate, it should be thoroughly stirred so that there are no particles. Next, you need to add hydrochloric acid in a thin stream to this solution. Do not forget about safety precautions and use gloves and goggles. After you have added hydrochloric acid to the solution, it should be thoroughly mixed.

So, the solution is ready and you can start copper plating at home. To do this, you need to take the metal part on which you are going to apply a layer of copper and prepare it for work. Preparation includes its processing sandpaper. This procedure allows not only to clean metal surface, but also degrease it. The same procedure will be relevant for parts made of brass or lead. After that, the coating must be thoroughly rinsed in a solution soda ash. This will allow the material to be more thoroughly degreased.

Soda ash for degreasing material

Next, the surface must be immersed in a solution of copper sulfate and of hydrochloric acid. It should be noted that the first layer of copper is very thin and weak, so it is advisable to remove it with a wire brush. After you have done this, the surface of the steel or lead should be washed again in the soda ash solution and again immersed in the copper plating solution. These manipulations will lead to the fact that the copper layer at home on the surface will be much thicker and much stronger, since it can be removed from the object only using sandpaper, and not a metal brush like last time.

This method allows you to make a very high-quality copper coating, which can only be removed with sandpaper. To improve the copper coating at home, the part should be immersed again in the solution. This method is characterized by its simplicity and high efficiency including lead products.

Copper plating procedure

Copper plating is usually called the procedure of galvanic deposition of copper, the thickness of the copper layer in such cases can be from 300 microns or more. Copper plating of steel is one of the most important processes in electroplating, as it is used as an additional process before applying other metals for chromium plating, nickel plating, silver plating.

The copper layer adheres perfectly to steel and is able to level various defects on the surface.

Copper coatings are characterized by high adhesion to other surfaces, lead products, especially metal ones, as well as high electrical conductivity and ductility. The newly applied coating is hot pink matte or glossy. Under the influence of atmospheric influences, copper coatings can oxidize, become coated with oxides with various stains rainbow look.

Areas of use for copper plating

Usually, galvanic copper plating can be used:

AT decorative purposes. Given the huge popularity at the present time of antique copper products. There are methods of artificial aging of steel products;
in electroplating. Widespread in the jewelry industry, among souvenirs, for making bas-reliefs, etc.;
In the tech industry. Copper plating is very important in the electrical field. The low cost of copper plating compared to gold or silver coatings reduces the cost of manufacturing electrodes, electrical busbars, contacts and other elements from lead steel.

Copper plating occurs together with the application of other electroplated coatings

If you need to apply a multi-layer protective and decorative coating on a layer of steel. In the vast majority of cases, copper is used here together with nickel and chromium. This improves adhesion to the base metal and obtains a high-strength, high-gloss coating;
To avoid cementation of the site. Copper plating of lead will avoid the appearance of carbonization on steel sections. For the application of the copper layer, only those areas where cutting will be carried out are used;
When performing restoration and restoration work. This method is most commonly used to restore chrome parts on cars and motorcycles. For these purposes, enough thick layer copper, of the order of 100-250 microns and more, which allows you to close all defects and damage to the metal for applying subsequent coatings;

Varieties of copper plating

Using electrolyte immersion;
Without immersion in electrolyte.

The first method involves processing metal product sandpaper, brush and rinse with water. Then degreasing in hot soda solution with re-wash. Next, two copper plates, anodes, are lowered into a glass container on copper wires. A part is suspended between the plates on a wire, after which a current is started.

The second method is relevant for steel, aluminum and zinc products.

Home copper plating

This procedure is relevant for various occasions, since copper coating can be used for aluminum cutlery, souvenirs, candle holders, etc. Non-metal items that have been coated with copper have a unique effect. These can be plant stems, leaves, etc. Due to the fact that there is no conductive layer in the coated objects, a special electrically conductive varnish is used instead, which is applied to the surface.

The composition of the varnish includes a number of organic solvents, foaming agents and finely dispersed graphite powder, due to which electrical conductivity is created. The varnish is applied in a thin layer on a dry surface, and after drying in an hour, you can start copper plating. If desired, copper can be given various color shades, using for this special ways. High quality and the uniqueness of such products is quite deservedly equated to real jewelry.

Video: Copper plating at home

You can master the technology of electroplating materials with metals quite quickly, this does not require extensive knowledge in physics or chemistry, but it is still desirable to have an idea of \u200b\u200bwhat you want to get "at the end" :)

The very principle of copper plating lies in the deposition of copper on the surface of the base object using electro-chemical processing of the semi-finished product form.

By by and large, affordable way coatings of forms with copper - only one.

The technology of copper plating of various non-metallic objects is called "galvanoplasty" ("electroforming" is the Western analogue of the name). It is quite possible to produce a wide variety of jewelry details using the electroforming method.

A necessary condition for obtaining a high-quality copper coating is the exact observance of the copper plating technology. So, let's move on to creating a copper plating installation.

Copper plating (copper plating) is carried out in containers that usually have a rounded or rectangular shape. Another form of container will also not interfere with the process. The volume of the container is determined by the size and number of products (objects) to be copper-plated. Perfectly glazed ceramic, glass or plastic containers.

You will need source direct current low voltage (from 2 to 6V). Rectifiers for charging batteries are ideal for this, and if one is not found in your household, then rechargeable batteries or batteries.

current strength you can adjust it using a slider rheostat (don't be alarmed, you can notice it with a suitable variable resistor, it looks like a volume control, such a knob or a slider).

You will also need multimeter to control the current strength (from 0 to 3-5 amperes) and to measure voltage. Typically, an ammeter chargers It is still present and does not need to be purchased separately.

For creating electrolyte stock up blue vitriol and sulfuric acid. Instead of sulfuric acid, an electrolyte, which is usually used in car batteries, is excellent.

To apply a conductive layer to the surface of the part to be copper-plated, you will need conductive varnish.

As anode can be used copper plates, and in the absence of such - the usual copper wire - vein mined from ordinary power cable which can be purchased at any electrical supply store. The area of this plate or wire should be approximately equal to the area of the object covered with copper, in any case - the larger it is, the better. The task of the anode is to slowly dissolve, giving copper into the solution.

It will also be necessary for coppering ethanol.

The electrolyte is prepared as follows:
230-250 grams of copper sulfate and 40-50 ml are taken per 1 liter of water. sulfuric acid (sulfuric acid can be purchased in the form of a composition called "Electrolyte" in auto and hardware stores).

* Be careful when handling electrolyte - it can damage skin, eyes and even clothing.

* It should be remembered that although the solutions do not contain highly toxic substances, they should still be handled with some care so as not to get poisoned or burned.

* It is advisable to store the solutions in a dark glass container with a well-closing lid (but a regular plastic bottle is also fine).

Modern technology puts forward stringent performance requirements structural elements, in many cases these problems are solved by chemical copper plating. The use of special coatings on the surfaces of parts is economically beneficial, since galvanic copper plating makes it possible to reduce the metal consumption of products made of expensive metals.

Physical and mechanical characteristics of copper and the scope of copper plating

The density of copper is 8.96 g/cm 3 , the atomic mass is 693.54, the electrical resistivity is 1.68×10 -8 Ohm×m, the melting point is +1083°C. Outdoors in the presence of aggressive chemical compounds copper oxidizes, upon contact with sulfur compounds, it becomes covered with a film of copper sulfide dark brown or shades of gray. Under the influence of carbon dioxide and moisture, the film acquires green color, the upper layer consists of bicarbonates. Copper dissolves easily in a solution of nitric acid, dilute sulfuric acid has almost no negative effect on chemical copper plating. But the presence of oxygen increases the rate of flow chemical reactions. In the presence of open pores in the coating, a galvanic couple is formed, which must be taken into account when copper plating. Iron in this case is the anode, corrosion processes proceed very intensively.

Due to such features, the copper plating process in most cases must be completed additional processing surfaces. Coatings are sanded or polished to a mirror finish. Copper has high adhesion with various metals: aluminum, silver, zinc, nickel, lead, chromium, etc. In connection with these features, chemical copper plating is often used to create a sublayer for silvering, nickel plating, chromium plating of parts surfaces. Copper plating has become widespread as a method effective protection separate sections of parts from the appearance of the effect of carburization during the carburizing process. Depending on the purpose of parts or products, galvanic copper deposition can have the following thickness:

Comparative indicators of solutions

In the copper plating process, a large number of special technological solutions divided into two large groups:

A simple acidic electrolyte. Of the simple ones, a fluoroborate, silicofluoride, sulfate, chloride and sulfamide solution is used.
complex electrolyte. Predominantly alkaline, copper is present as positively or negatively charged complex ions.

The deposition process in acidic electrolytes occurs when high density current, they are stable, simple in chemical composition. The main components are the corresponding acids and salts, copper deposits from them are quite dense and have a coarse-grained structure. Disadvantages - direct copper plating of steel, zinc alloys and other metals occurs with a lower negative potential than copper.

Processing of parts in complex electrolytes is carried out at the expense of complex ions, they require high cathodic polarization. The current output is less, which contributes to a more uniform deposition, the structure is finely crystalline. Pyrophosphate, cyanide, ammonium, tripolyphosphate, citrate and other solutions are used.

Simple acid compounds

Sulfate. The main components are sulfuric acid and copper sulfate. Sulfuric acid compound is characterized by low electrical conductivity; sulfuric acid is added to increase the parameter. The current yield of copper reaches 100%, no hydrogen is released at the cathode. By increasing the concentration of acid, the solubility of sulfate decreases, which lowers the upper limit of the maximum allowable current density.

With stirring, the concentration of copper ions on the cathode layer increases. With an increase in temperature, the solubility of copper sulfate increases, the electrolyte increases the acidity, which leads to the formation of fine crystalline precipitates.

Surfactants are added to the electrolyte to improve cathodic polarization. Additionally, they reduce the formation of build-up on sharp edges.

For the formation of a brilliant coating, AMF anodes that do not allow the formation of sludge, or anodes from highly pure refined copper are used.

Fluoroborate electrolyte.

To prevent the ingress of sludge, the anodes are placed in covers made of acid-resistant material, in addition, the solution is constantly filtered.

Fluoroborate. They are highly resistant, the plating is dense and finely crystalline, the scattering characteristics are the same as with sulfate copper plating. Due to the high solubility, the current density increases; it is impossible to deposit copper directly on the parts.

With continuous stirring, it is allowed to increase the current density. Control of technological parameters of copper plating is carried out by measuring the acidity of the solution. Sodium carbonate is used to improve the quality of copper plating, and copper sulphate is used to lower it.

Nitrate. The electrolyte is used in electroforming, provides higher quality draft.

Modes and composition of nitrate electrolytes

Complex electrolytes

Cyanide. The processing conditions are significantly different from acid precipitation, in which copper exists in the form of complex ions, which significantly reduces its activity. An increase in current density forces the cathode potential to shift sharply in the field negative values. But the copper plating process cannot be carried out at an increased current density due to the fact that the copper yield can drop to zero. The main components of the solution are free sodium cyanide and complex potassium cyanide. During operation, the copper content decreases due to their insufficient solubility.

Mode and composition of cyanide electrolytes for copper plating

Pyrophosphate. Copper deposits have a fine-grained structure, smooth, shiny or semi-shiny. To improve the quality of processing and increase the cathode and anode densities, copper sulfate can be added. The cathodic potential in pyrophosphate solutions has more negative parameters than in acid ones.

Mode and composition of pyrophosphate electrolytes

Ethylenediamine. The copper plating process can be carried out directly on the steel surface; at low current densities, the cathodic polarization reaches large values. Scattering characteristics are higher than those of sulfate, but lower than that of a cyanide solution.

Mode and composition of ethylenediamine electrolytes

Polyethylenepolyamine. During the processing of parts, the potentials are shifted in the field of negative values, the electrolyte is used instead of cyanide.

Ammonium. The composition includes ammonia, ammonium sulfate and copper sulfate. At low current densities, the current efficiency decreases, copper plating is improved by adding ammonium nitrate. Precipitation is uniform in thickness, dense and semilustrous.

Without special surface treatment, copper deposits have insufficient adhesion, the reason is the passivation of steel with an ammonia solution. Improving the parameters of the coating is achieved by introducing copper nitrate into the solution.
Copper plating bath device Linear parameters and design features must meet the requirements of GOST 23738-85. is made of modified highly resistant plastics, specific grades are selected taking into account the parameters of technological processes.

Bathtub without pocket

Bathtub with pocket

The specific choice of a copper plating bath is carried out depending on the characteristics of the enterprise, the characteristics of the parts to be copper plated and the overall production capacity.

During the design, the maximum loads are calculated taking into account the volume of the solution, the length, height and width can be changed at the request of customers. If necessary, it is installed on copper plating baths optional equipment and plumbing fittings. Due to special mechanisms, the quality of the copper plating process is improved. The plastics used are adapted to the chemical composition of the electrolyte and temperature conditions copper plating.

Mechanical surface preparation

Before copper plating, scale, burrs and shells must be removed from the surface. The quality of processing is regulated by the provisions of the current GOST 9.301-86. Specific roughness parameters are set depending on the purpose of the coating. After machining parts from the surface must be removed all defects that have Negative influence on the quality of copper plating. AT without fail technical lubricant and emulsion, metal shavings, products of corrosion processes and dust are removed.

Preparation for copper plating is carried out during the following technological operations:

Grinding. Upper layer parts are removed with abrasive elements, it can be thin, decorative or rough.
Polishing. During the operation, the smallest protrusions are smoothed out, the surface is shiny and mirror-like.
Brushing. Metal brushes are used to clean surfaces.
Tumbling. Details are rolled in special bells.
Chemical and electrochemical degreasing. For processing, organic and inorganic solutions are used.

From quality pre-training surfaces largely depends on the process of copper plating and physical indicators of precipitation.

In order to prepare a metal object for further processing, it must be coated with copper - this process is called electroplating. The principle of its operation is to deposit on the surface of the workpiece from another metal, which is dissolved suitable remedy. From this article, you will learn how to copper-plated metal at home in order to realize all your subsequent ideas with a workpiece.

What is it for?

Electroplating technology includes the creation of a solution and the formation of various electrodes. During this process, the copper ions that are dissolved in the electrolyte are attracted by the minus pole of the workpiece to their surface point.

Electroplating of metal parts in industry is used not only as a final processing process. It can be used to prepare parts for a subsequent operation (for example, chrome plating, nickel plating, silver plating of any items).

At home, chemical copper plating of parts is most often carried out. It is also worth noting that there are many ways to carry out such a procedure, each of which has its own advantages and disadvantages.

Spheres of use of electroplating

Copper-plating metal at home has recently become very useful. Most often, this procedure is suitable for the following purposes:

In decorative. For example, various details are aged in this way in order to give a vintage look.
in electroplating. Very often put a copper layer on souvenirs, jewelry.
In the technology industry. This is due to the low cost of copper plating.
For applying a protective and decorative layer of a product.
In order to restore or restore any parts.

Electroplating with electrolyte solution

In this way, you can copper steel at home, as well as other parts with the exception of aluminum and zinc.

For the further procedure, it is necessary to prepare the following materials:

Small plates of copper.
Conductive wire (it is better to stock up on a few meters).
Current source, the voltage of which should not be more than 6 V.
Rheostat to regulate the current.
Ammeter.
Electrolyte.

Important! The last component can be purchased at a special store, or you can make it yourself. To do this, you need to make a solution of sulfuric acid and distilled water at a ratio of 2/100 ml. The right solution it will turn out when you add no more than 20 g of copper sulfate to the composition.

The essence of the work is as follows:

Clean the workpiece with sandpaper - this is necessary in order to remove the oxide film. Further, it will not be so difficult to cover metal with copper at home.
Degrease a metal object with a hot soda solution, rinse it with clean water.
Pour the prepared electrolyte into a suitable glass container.
Dip two copper plates into the liquid on the prepared conductive wires. Between two plates of copper, an already degreased part is suspended on the same wiring.

Important! Make sure that the copper plates are completely immersed in the electrolyte.

Next, the ends of the wires from the copper plates are connected to the positive terminal of the current source, and the part to the negative. Next, an ammeter with a rheostat must be connected to the created electrical network. After the current is connected to the circuit, the rheostat needs to set it to about 15 mA per 1 cm of the detailed surface area.
Next, the part is aged for about 15-20 minutes.
After the time has elapsed, you need to turn off the power supply and pull the metal being processed out of the solution.

In the end, you will notice that you were able to copper the metal at home, because it will be completely covered with a thin shiny film.

Important! The thickness of the copper layer will depend on how long the part was kept in the electrolyte.

Electroplating in a different way

This method is perfect for the exception metals that were mentioned earlier - zinc and aluminum.

Work algorithm:

Arm yourself with a stranded copper wire, remove the insulation from both ends.
Rub the soft wire on one side, getting a simulated brush. Tie an object that looks like a handle to this end.
The opposite side of the wire must be connected to the positive terminal of the power source.

Important! The voltage must be no more than 6 V.

Prepare the electrolyte in the manner described above, pour it into a wide container - this is necessary for the convenience of dipping the “brush” there.
Place a small piece of metal inside the prepared container. Connect it, using a wire, to the negative terminal of the current source, also with a voltage of 6 V.

Important! The part must be pre-cleaned and degreased.

The process of coating aluminum with copper at home is as follows: the cystic end of the wire must be blotted with an electrolyte solution and run over the surface of the treated metal object without touching it. Further, the negatively charged metal will attract copper ions to itself and its surface will acquire a copper film.

Important! Make sure that there is a small layer of electrolyte solution between the end of the torn wire and the metal (both the cathode and the anode must be moistened with this solution).

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