-: MAKING A DRY CELL :-

A dry cell is a portable battery used in torches, transistor radio sets, calculators, digital watches, etc. as a source of electricity. A conventional dry cell consists of a cylindrical vessel of zinc with a carbon rod in its centre. A paste of manganese dioxide and ammonium chloride is filled in it. When the zinc vessel and carbon rod are connected to a bulb by means of a wire, the bulb glows due to the flow of current. The voltage of such a cell is about 1.5 volt. You can make a model of such a dry cell as follows. 

*** MATERIALS REQUIRED ***

•  Take some water and mix starch in it. Boil the mixture till the starch forms a thick paste. Mix sufficient quantity of manganese dioxide with the paste of starch, making a very thick paste of manganese dioxide.

• Take the zinc plate and uniformly spread the manganese dioxide paste over it. 

•  Take some cotton wool, spread it to the size of the zinc plate and soak it in the saturated solution of ammonium chloride. Put it over the layer of paste. Put another layer of manganese dioxide paste over it.

•  Now, put a carbon plate on the above layer of manganese dioxide. Your dry cell Is ready for use.

•  Take two copper wires. Connect their two ends to two crocodile clips and other two ends to a bulb holder. Clip the zinc plate and carbon plate with the two crocodile clips separately. The electric current will flow through the bulb and the bulb will glow.

NOTE :- 

There are three main types of dry cell batteries: Carbon-zinc, alkaline and
mercury. The one described here is a carbon-zinc dry cell.
An alkaline dry cell battery is more powerful. It lasts five to eight times longer than a carbon-zinc battery. It has a carbon electrode and a zinc casing electrode. The electrolyte is a strong alkali solution, potassium
hydroxide. Alkaline dry cells are used mainly for portable radios.

In a mercury dry cell, the voltage remains constant to the end of the battery’s life. These are used in toy cars, electric razors, deaf aids etc. Electric toothbrushes often use this type of cells. In this cell, a mercuric
oxide electrode is used. The other electrode is the zinc casing. The
electrolyte is potassium hydroxide. Nickel cadmium batteries though wet
cell batteries can be sealed air tight and are ideal for use in portable tools
and equipments. 

-: MAKING A TRAFFIC LIGHTS :-

Traffic lights are used in big cities at major road crossings to make the vehicular movement smooth and avoid accidents. The red light is a signal of ‘stop’, green light is a signal of ‘go’ and orange light is a signal of ‘ready’. These lights are operated automatically. You can make a model of the traffic lights: 

*** MATERIALS REQUIRED ***

• Take two wooden strips-one (12 X 4 X 1)inch and the other (8 X 4 X 1)inch. Put them at right angles and fix them with the help of wood screws. 
• Put three bulb holders in a line and wire them in parallel. Insert the bulbs in the holders: top - red, middle - orange and bottom- green. 

• Fix the three wires from three bulbs at the base with drawing pins and put a fourth pin by pressing a U-clip under it. One end of it should be straightened so that it can be pressed against either of the three pins. This will act as a switch. 

• Connect the two dry cells in series, i.e. the positive terminal to negative of the other, to each bulb. 

• Connect the common wire from the bulbs to the positive terminal of the dry cell combination. Connect one wire from the negative terminal of the dry cell combination to the U-clip. Your model Is ready.
• Now, when you press the straightened edge of the clip against any of the three pins, one of the three bulbs will glow. 

-: PRODUCING ELECTRICITY FROM PATATOES :-

A primary voltaic cell makes use of a dilute solution of sulphuric acid in which one plate each of copper and zinc Is dipped. The electricity in this cell Is produced due to the migration of electrons from the zinc plate to copper plate in the dilute sulphuric acid solution. In fact, dilute sulphuric acid provides the path for the movement of electrons. In a similar way, potatoes act like the solution of dilute sulphuric acid between the copper and zinc strips which are used for producing electricity from potatoes.

*** MATERIALS REQUIRED ***

• Take 8 medium-sized potatoes and insert a strip of zinc on one end of each potato. Similarly, insert one copper strip on another end of each potato. 

• Arrange these potatoes on a table in two rows, each containing four potatoes.Connect the zinc strip of one potato to the copper strip of the other with the copper wire. Connect all zinc and copper strips with the help of the copper wire. 

• Now, take one wire connected to the zinc strip and one wire from the copper strip and connect these two with the 4.5 volt bulb. You will see that the bulb Immediately glows because the whole assembly acts as a battery. The bulb will glow only for a limited time. 

-: MAKING A KALEIDOSCOPE :-

It is an optical instrument that forms beautiful patterns, It contains two mirrors which meet at an angle of 600. Light from coloured objects placed near the mirrors-bounces between them to form several reflections. The most fascinating aspect about the kaleidoscope is that you never get the same pattern twice. Designers of wall papers and fabrics make use of this instrument to get ideas for new patterns.

*** MATERIALS REQUIRED ***

• Take two rectangular strips of mirrors identical in size about 8 inches long and 2 inches wide. Take one cardboard piece of the same size as the mirror strips. Join the mirrors and the cardboard together to form a long triangular tube. The slivered sides of the mirrors should face inward. The angles between them will be 600. This angle will produce six symmetrical patterns.

• Cut a piece of tracing paper or plastic paper to fit over one end of the triangular tube and glue or tape It in place. 

• Take about 7-8 pieces of bangles of different colours. Put these coloured pieces In the tube. Then hold the tube upright so that the pieces rest on the tracing paper. 

•  Look down into the tube and see the pattern formed. The end of the triangular tube near the eye can be closed by a transparent plastic sheet or glass sheet. The pattern will consist of 6 IdentIcal sections. Shake the kaleidoscope to change the patterns.

 

-: MAKING A SLIDE PROJECTOR :-

A slide projector is an optical instrument used to see slides on a screen in an enlarged image. It makes use of a good quality magnifier and a light source. When light falls on the slide, its enlarged image is projected on a screen. This simple project will demonstrate the principle of projection. 

-: MATERIALS REQUIRED :-

• Take one magnifying lens of about 3 inch diameter and 50 cm focal length.
• Take one cardboard box with a deep lid and of the same height as that of the focal length of the lens. The other dimensions of the box are not important. In the center of the lid of the cardboard box, cut a hole to mount the lens. Then glue the lens in place 

• At the centre of the bottom of the box, cut a hole slightly larger than your slides.       
•  For the rear part of the projector, take the second box and make a hole in the bottom of this box of the same size as the hole in the front box. The two holes should line up with each other when the boxes are placed on their sides.

                     

• Join the two boxes by gluing the pieces of cardboard to the sides. Leave a small gap between the boxes for the slide carrier. 

                  

• Make the slide holder from a piece of thick cardboard. This is used to position a slide between the holes in the boxes. 

                        

                 

•  Set slide between two box.

• In a darkened room, project the slide on to a white paper or white wall by switching on the bulb. To get a sharp picture, adjust the focus by pulling out or pushing in the deep lid that holds the lens. 

               

-: MAKING A PERISCOPE :-

A periscope is an optical instrument with which a person can see around corners and other obstractions. It consists of a long tube with a reflecting mirror or prism at each end. The reflecting surfaces are parallel to one another. They are arranged at an angle of 45⁰  inside the tube. Periscopes are important instruments used in submarines and tanks. They are also used by soldiers in the battlefield.

-: MATERIALS REQUIRED :-

• Take a cardboard tube and two small mirrors. Use a small hacksaw to cut a section near each end of the tube. First make a horizontal cut to about 3/4 of the diameter of the tube. Now make two cuts at 45⁰ to the horizontal cuts. Use a 45⁰  set-square for guidance while making these cuts. 

• Glue the mirrors in places where the sloping cuts have been made. The silvered surfaces of the mirror should face each other so that light can pass through the instrument. 

-: MAKING DANCING OR DISCO LIGHTS :-

This type of dancing or disco lights are commonly seen in the movies or clubs.
These lights have charming colours dancing on the shirts or coats of performers.
You can make the dancing light with the help of this project. This is an inexpensive
project and easy to make.

*** MATERIALS REQUIRED ***       

•  Connect all the components on solderless breadboard as shown in the IMAGE. 
•  The positive wire of first LED in each series should be connected to the positive terminal of the battery. 
• After assembling all the components as per the circuit, as soon as you connect the battery, you will see the charming green and red colours. You can put a switch In the battery circuit. 

Try to make it on a small PCB so that it can be pinned with the pocket of the shirt.