Chapter IV
that chemical action may be used to produce electricity and that in the case of a cell such as that invented by Volta, the zinc electrode gradually wastes away and finally enters into solution in the sulphuric acid.
It is possible exactly to reverse this action and to produce what is known as _electrolysis_. If an electrolyte in which a metal has been "dissolved" is properly arranged so that a current of electricity may be passed through the solution, the metal will "plate out," or appear again upon one of the electrodes.
Electrolysis makes possible electro-plating and thousands of other exceedingly valuable and interesting chemical processes.
More than one-half of all the copper produced in the world is produced _electrolytically_.
Practically all plating with gold, silver, copper and nickel is accomplished with the aid of electricity.
These operations are carried out on a very large scale in the various factories, but it is possible to reproduce them in any boy’s workshop or laboratory, with very simple equipment.
The proper chemicals, a tank, and a battery are the only apparatus required. The current must be supplied by storage cells or a bichromate battery because the work will require five or six amperes for quite a long period.
A small rectangular glass jar will make a first class tank to hold the electrolyte.
The simplest electro-plating process, and the one that the experimenter should start with is copper-plating.
Fill the tank three-quarters full of pure water and then drop in some crystals of copper-sulphate until the liquid has a deep blue color and will dissolve no more.
Obtain two copper rods and lay them across the tank. Cut two pieces of sheet copper having a tongue at each of two corners so that they can be hung in the solution, as shown in Figure 313. Hang both of the sheets from one of the copper rods. Connect this rod to the _positive_ pole of the battery. These sheets are known as the anodes.
Then if a piece of carbon, or some metallic object is hung from the other rod and connected to the _negative_ pole of the battery, the electro-plating will commence. The apparatus should be allowed to run for about half an hour and then the object hung from the rod connected to the negative pole of the battery should be lifted out and examined. It will be found thickly coated with copper. It is absolutely necessary to have the poles of the battery connected in the manner stated, or no deposit of copper will take place.
Objects which are to be electro-plated must be free from all traces of oil or grease and absolutely clean in every respect, or the plating will not be uniform, because it will not stick to dirty spots.
[Illustration: Fig. 313.—A Glass Jar arranged to serve as an Electro-Plating Tank.]
Such articles as keys, key-rings, tools, etc., can be prevented from rusting by coating with nickel.
Nickel-plating is very similar to copper-plating. Instead, however, of having two copper sheets suspended from the rod connected to the positive pole of the battery, they must be made of nickel.
The electrolyte is composed of one part of nickel-sulphate dissolved in twenty parts of water to which one part of sodium-bisulphate is added.
This mixture is placed in the tank instead of the copper-sulphate. The objects to be plated are hung from the copper rod connected to the negative pole of the battery.
When the nickel-plated articles are removed from the bath they will have a dull, white color known as "white nickel." When white nickel is polished with a cloth wheel revolving at high speed, and known as a buffing-wheel, it will assume a high luster.
HOW TO MAKE A RHEOSTAT
It is often desirable to regulate the amount of current passing through a small lamp, motor, or other electrical device operated by a battery.
This is accomplished by inserting resistance into the circuit. A rheostat is an arrangement for quickly altering the amount of resistance at will.
A simple rheostat is easily made by fitting a five-point switch such as that shown in Figure 95 with several coils of German-silver resistance wire. German silver has much more resistance than copper wire, and is used, therefore, because less will be required, and it will occupy a smaller space.
A five-point switch will serve satisfactorily in making a rheostat, but if a finer graduation of the resistance is desired it will be necessary to use one having more points.
Two lines of small wire nails are driven around the outside of the points, and a German-silver wire of No. 24 B. & S. gauge wound in zig-zag fashion around the nails from one point to the other.
[Illustration: Fig. 314.—A Rheostat.]
The rheostat is placed in series with any device it is desirable to control. When the handle is on the point to the extreme left, the rheostat offers no resistance to the current. When the lever is placed on the second point, the current has to traverse the first section of the German-silver wire and will be appreciably affected. Moving the handle to the right will increase the resistance.
If the rheostat is connected to a motor, the speed can be increased or decreased by moving the lever back and forth.
In the same manner, the light from a small incandescent lamp may be dimmed or increased.
A CURRENT REVERSER OR POLE-CHANGING SWITCH
A pole-changing or current reversing switch is useful to the experimenter. For example, if connected to a small motor, the motor can be made to run in either direction at will. A motor with a permanent magnet field can be reversed by merely changing the wires from the battery so that the current flows through the circuit in the opposite direction. If the motor is provided with a field winding, however, the only way that it can be made to run either way is by reversing the field. This is best accomplished with a pole-changing switch.
Such a switch may be made by following the same general method of construction as that outlined on pages 107 and 108, but making it according to the design shown in Figure 315.
Motors such as those illustrated can be made to reverse by connecting to a pole-changing switch in the proper manner.
The two outside points or contacts (marked _D_ and _D_) should both be connected to one of the brushes on the motor. The middle contact, _C_, is connected to the other brush.
One terminal of the field is connected to the battery. The other terminal of the field is connected to the lever, _A_. _B_ connects to the other terminal of the battery.
[Illustration: Fig. 315.—A Pole-Changing Switch or Current Reverser. The Connecting Strip is pivoted so that the Handle will operate both the Levers, A and B.]
When the switch handle is pushed to the left, the lever _A_ should rest on the left-hand contact, _D_. The lever _B_ should make contact with _C_. The motor will then run in one direction. If the handle is pushed to the right so that the levers _A_ and _B_ make contact respectively with _C_ and _D_ (right-hand), the motor will reverse and run in the opposite direction.
A COMPLETE WIRELESS RECEIVING SET
Many experimenters may wish to build a wireless receiving set which is permanently connected and in which the instruments are so mounted that they are readily portable and may be easily shifted from one place to another without having to disturb a number of wires.
The receiving set shown in Figure 316 is made up of some of the separate instruments described in