Electricity for Boys Part 10

PLATING IRON WITH COPPER.--We have room in this chapter for only one concrete example of this work, which, with suitable modifications, is an example of the art as practiced commercially. Iron, to a considerable extent, is now being coated with copper to preserve it from rust. To carry out this work, however, an electroplating dynamo, of large amperage, is required, the amperage, of course, depending upon the surface to be treated at one time. The pressure should not exceed 5 volts.

The iron surface to be treated should first be thoroughly cleansed, and then immediately put into a tank containing a cyanide of copper solution. Two forms of copper solution are used, namely, the cyanide, which is a salt solution of copper, and the sulphate, which is an acid solution of copper. Cyanide is first used because it does not attack the iron, as would be the case if the sulphate solution should first come into contact with the iron.

A sheet of copper, termed the anode, is then placed within the tank, parallel with the surface to be plated, known as the cathode, and so mounted that it may be adjusted to or from the iron surface, or cathode.

A direct current of electricity is then caused to flow through the copper plate and into the iron plate or surface, and the plating proceeded with until the iron surface has a thin film of copper deposited thereon. This is a slow process with the cyanide solution, so it is discontinued as soon as possible, after the iron surface has been completely covered with copper. This copper surface is thoroughly cleaned off to remove therefrom the saline or alkaline solution, and it is then immersed within a bath, containing a solution of sulphate of copper. The current is then thrown on and allowed so to remain until it has deposited the proper thickness of copper.

DIRECTION OF CURRENT.--If a copper and an iron plate are put into a copper solution and connected up in circuit with each other, a primary battery is thereby formed, which will generate electricity. In this case, the iron will be positive and the copper negative, so that the current within such a cell would flow from the iron (in this instance, the anode) to the negative, or cathode.

The action of electroplating reverses this process and causes the current to flow from the copper to the iron (in this instance, the cathode).

CHAPTER XIV

ELECTRIC HEATING, THERMO ELECTRICITY

GENERATING HEAT IN A WIRE.--When a current of electricity pa.s.ses through a conductor, like a wire, more or less heat is developed in the conductor. This heat may be so small that it cannot be measured, but it is, nevertheless, present in a greater or less degree. Conductors offer a resistance to the pa.s.sage of a current, just the same as water finds a resistance in pipes through which it pa.s.ses. This resistance is measured in ohms, as explained in a preceding chapter, and it is this resistance which is utilized for electric heating.

RESISTANCE OF SUBSTANCES.--Silver offers less resistance to the pa.s.sage of a current than any other metal, the next in order is copper, while iron is, comparatively, a poor conductor.

The following is a partial list of metals, showing their relative conductivity:

Silver 1.

Copper 1.04 to 1.09 Gold 1.38 to 1.41 Aluminum 1.64 Zinc 3.79 Nickel 4.69 Iron 6.56 Tin 8.9 Lead 13.2 German Silver 12.2 to 15

From this table it will be seen that, for instance, iron offers six and a half times the resistance of silver, and that German silver has fifteen times the resistance of silver.

This table is made up of strands of the different metals of the same diameters and lengths, so as to obtain their relative values.

SIZES OF CONDUCTORS.--Another thing, however, must be understood. If two conductors of the same metal, having different diameters, receive the same current of electricity, the small conductor will offer a greater resistance than the large conductor, hence will generate more heat. This can be offset by increasing the diameter of the conductor. The metal used is, therefore, of importance, on account of the cost involved.

COMPARISON OF METALS.--A conductor of aluminum, say, 10 feet long and of the same weight as copper, has a diameter two and a quarter times greater than copper; but as the resistance of aluminum is 50 per cent.

more than that of silver, it will be seen that, weight for weight, copper is the cheaper, particularly as aluminum costs fully three times as much as copper.

[Ill.u.s.tration: _Fig. 96._ SIMPLE ELECTRIC HEATER]

The table shows that German silver has the highest resistance. Of course, there are other metals, like antimony, platinum and the like, which have still higher resistance. German silver, however, is most commonly used, although there are various alloys of metal made which have high resistance and are cheaper.

The principle of all electric heaters is the same, namely, the resistance of a conductor to the pa.s.sage of a current, and an ill.u.s.tration of a water heater will show the elementary principles in all of these devices.

A SIMPLE ELECTRIC HEATER.--In Fig. 96 the ill.u.s.tration shows a cup or holder (A) for the wire, made of hard rubber. This may be of such diameter as to fit upon and form the cover for a gla.s.s (B). The rubber should be 1/2 inch thick. Two holes are bored through the rubber cup, and through them are screwed two round-headed screws (C, D), each screw being 1-1/2 inches long, so they will project an inch below the cap.

Each screw should have a small hole in its lower end to receive a pin (E) which will prevent the resistance wire from slipping off.

The resistance wire (F) is coiled for a suitable length, dependent upon the current used, one end being fastened by wrapping it around the screw (C). The other end of the wire is then brought upwardly through the interior of the coil and secured in like manner to the other screw (D).

Caution must be used to prevent the different coils or turns from touching each other. When completed, the coil may be immersed in water, the current turned on, and left so until the water is sufficiently heated.

[Ill.u.s.tration: _Figs. 97-98._ RESISTANCE DEVICE]

HOW TO ARRANGE FOR QUANt.i.tY OF CURRENT USED.--It is difficult to determine just the proper length the coil should be, or the sizes of the wire, unless you know what kind of current you have. You may, however, rig up your own apparatus for the purpose of making it fit your heater, by preparing a base of wood (A) 8 inches long, 3 inches wide and 1 inch thick. On this mount four electric lamp sockets (B). Then connect the inlet wire (C) by means of short pieces of wire (D) with all the sockets on one side. The outlet wire (E) should then be connected up with the other sides of the sockets by the short wires (F). If, now, we have one 16-candlepower lamp in one of the sockets, there is a half ampere going through the wires (C, F). If there are two lamps on the board you will have 1 ampere, and so on. By this means you may readily determine how much current you are using and it will also afford you a means of finding out whether you have too much or too little wire in your coil to do the work.

[Ill.u.s.tration: _Fig. 99._ PLAN VIEW OF ELECTRIC IRON]

AN ELECTRIC IRON.--An electric iron is made in the same way. The upper side of a flatiron has a circular or oval depression (A) cast therein, and a spool of slate (B) is made so it will fit into the depression and the high resistance wire (C) is wound around this spool, and insulating material, such as asbestos, must be used to pack around it. Centrally, the slate spool has an upwardly projecting circular extension (D) which pa.s.ses through the cap or cover (E) of the iron. The wires of the resistance coil are then brought through this circular extension and are connected up with the source of electrical supply. Wires are now sold for this purpose, which are adapted to withstand an intense heat.

[Ill.u.s.tration: _Fig. 100._ SECTION OF ELECTRIC IRON]

The foregoing example of the use of the current, through resistance wires, has a very wide application, and any boy, with these examples before him, can readily make these devices.

THERMO ELECTRICITY.--It has long been the dream of scientists to convert heat directly into electricity. The present practice is to use a boiler to generate steam, an engine to provide the motion, and a dynamo to convert that motion into electricity. The result is that there is loss in the process of converting the fuel heat into steam; loss to change the steam into motion, and loss to make electricity out of the motion of the engine. By using water-power there is less actual loss; but water-power is not available everywhere.

CONVERTING HEAT DIRECTLY INTO ELECTRICITY.--Heat may be converted directly into electricity without using a boiler, an engine or a dynamo, but it has not been successful from a commercial standpoint. It is interesting, however, to know and understand the subject, and for that reason it is explained herein.

METALS; ELECTRIC POSITIVE-NEGATIVE.--To understand the principle, it may be stated that all metals are electrically positive-negative to each other. You will remember that it has hereinbefore been stated that if, for instance, iron and copper are put into an acid solution, a current will be created or generated thereby. So with zinc and copper, the usual primary battery elements. In all such cases an electrolyte is used.

Thermo-electricity dispenses with the electrolyte, and nothing is used but the metallic elements and heat. The word thermo means heat. If, now, we can select two strips of different metals, and place them as far apart as possible--that is, in their positive-negative relations with each other, and unite the end of one with one end of other by means of a rivet, and then heat the riveted ends, a current will be generated in the strips. If, for instance, we use an iron in conjunction with a copper strip, the current will flow from the copper to the iron, because copper is positive to iron, and iron negative to copper. It is from this that the term positive-negative is taken.

The two metals most available, which are thus farthest apart in the scale of positive-negative relation, are bis.m.u.th and antimony.

[Ill.u.s.tration: _Fig. 101._ THERMO-ELECTRIC COUPLE]

In Fig. 101 is shown a thermo-electric couple (A, B) riveted together, with thin outer ends connected by means of a wire (C) to form a circuit.

A galvanometer (D) or other current-testing means is placed in this circuit. A lamp is placed below the joined ends.

THERMO-ELECTRIC COUPLES.--Any number of these couples may be put together and joined at each end to a common wire and a fairly large flow of current obtained thereby.

One thing must be observed: A current will be generated only so long as there exists a difference in temperature between the inner and the outer ends of the bars (A, B). This may be accomplished by water, or any other cooling means which may suggest itself.

CHAPTER XV

ALTERNATING CURRENTS, CHOKING COILS, TRANSFORMERS, CONVERTERS AND RECTIFIERS

DIRECT CURRENT.--When a current of electricity is generated by a cell, it is a.s.sumed to move along the wire in one direction, in a steady, continuous flow, and is called a _direct_ current. This direct current is a natural one if generated by a cell.

ALTERNATING CURRENT.--On the other hand, the natural current generated by a dynamo is alternating in its character--that is, it is not a direct, steady flow in one direction, but, instead, it flows for an instant in one direction, then in the other direction, and so on.

A direct-current dynamo such as we have shown in Chapter IV, is much easier to explain, hence it is ill.u.s.trated to show the third method used in generating an electric current.

It is a difficult matter to explain the principle and operation of alternating current machines, without becoming, in a measure, too technical for the purposes of this book, but it is important to know the fundamentals involved, so that the operation and uses of certain apparatus, like the choking coil, transformers, rectifiers and converters, may be explained.

THE MAGNETIC FIELD.--It has been stated that when a wire pa.s.ses through the magnetic field of a magnet, so as to cut the lines of force flowing out from the end of a magnet, the wire will receive a charge of electricity.

[Ill.u.s.tration: _Fig. 102._ CUTTING A MAGNETIC FIELD]