-: 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.