Category Science & Technology

          A siren is a device basically used to make a warning or to give a signal. It produces a loud, piercing sound of a definite pitch. It is powered by electric motors, steam or hand cranks.

          Sirens are used for various purposes, for example, in factories warning signal for any danger or accident; for cautioning people in case of enemy attacks; for traffic clearance by the vehicles of emergency services etc.

          Siren was invented in the late 18th century by a Scottish natural philosopher, John Robinson. It was named thus by a French engineer, Charles Cognaird de La Tour, who devised an acoustical instrument of the type in 1819 which is not generally used nowadays.

          The Cognaird siren consists of a cylindrical hollow box ‘C’, known as the wind chest. It has a fixed top lid ‘B’ that consists of a number of equidistant slanting holes arranged in circles. The chest is fixed at its lower end with a tube ‘T’, through which air can be blown in it by means of bellows. Another disc ‘D’ of the same size, with equal number of equidistant holes slanting in the opposite direction covers the first disc. This disc can be rotated by a mechanical arrangement. The air under pressure is forced into the wind chest of the siren due to which the disc begins to rotate. The air passes through the holes in the lower disc, and comes out through the holes in the upper disc. Every time the holes in the upper disc are above the holes in the lower disc, a puff of air escapes. This creates a sound. The pitch of the sound depends upon the speed at which the disc rotates.

          Now improved versions of this siren are available. One of such types of siren uses two small cylinders one inside the other, which have holes in them. An electric motor rotates the outer cylinder. Another device forces air or steam through the cylinders. The holes in the cylinders are actually small slots which are cut on a slant in lines encircling the cylinders. The air enters the slots in the first cylinder, and then passes through the slots in the second cylinder. Every time the slots are exactly opposite to one another, a sound is created by this movement of air through the slots. The faster the rotation of the outer cylinder, the louder the sound. 

               Anodizing is a process of coating a metal with a protective oxide layer on the surface to resist the corrosion of the metal. This can be done by either chemical or electrolytic means. In this regard, mainly aluminium or magnesium is anodized. However, metals like beryllium, tantalum, titanium and zinc can also be anodized. 

               The natural oxide film on aluminium is thin. Anodizing makes a thicker oxide layer. This protects the aluminium from corrosion and makes it last longer.

               In anodizing, aluminium is used as the positively charged electrode of an electrolytic cell. Electrolytes such as sulphuric acid or chromic acid are used as a solution. The oxide layer forms from the metal surface outside. This makes the outside layer slightly rough and porous. After anodizing, the pores on the metal are sealed by hot water or steam treatment which causes dehydration and results in the expansion of volume of the oxide. This further prevents corrosive substances from attacking the metal.

               Anodizing with sulphuric acid makes a clear oxide film. With chromic acid, a dull film is produced. Even coloured films can be produced by using dyes. Chromic acid is also used for anodizing zinc.

            A turbine is a machine used to convert energy generated by a moving liquid, gas or air into work. For example, the energy produced by fast flowing water is made to spin a rotating shaft by pushing on the angled blades set around a wheel mounted on the shaft, and this action produces the required work. The generated work helps to drive machines like propeller of a ship or an aeroplane or electric generators to produce electricity. The gas or liquid used in a turbine is known as the working medium. The working medium can be water, steam, gas or air.

            The water turbines use water from a waterfall or a dam to drive the turbine. There are two different methods to make the turbine wheels rotate. A water jet may be directed onto the blades. This is called a reaction turbine. In the second method, the turbine wheels are submerged in the flowing water. These turbines are used at hydro electric power stations. 

            For a steam turbine to operate, first the steam must to be produced by heating water in boilers. The steam enters the turbine at a high pressure. Inside the turbine, the pressure drops and the steam expands. This expansion drives the wheels around. Steam turbines have a series of wheels, mounted on the same shaft. This is because the steam expands gradually as it moves through the turbine. 

Continue reading “How does a turbine work?”

            A battery torch is a portable electrical appliance used for lighting. It has a variety of uses. Doctors use it to examine closely the ears, eyes, noses and mouths of the patients. Do you know how a torch produces light?

            A battery torch can be divided into four main parts: the battery, bulb, case and the switch. The battery is in the form of dry cells. Dry battery cells such as those in torches, transistors or calculators produce electricity only for a limited time. Most of the torches make use of two or three dry cells. The body of the torch is fitted with a bulb and dry cells. When the switch of the torch is pressed, the cells get connected to the bulb, and as a result of this the bulb emits light.

            The torch produces light by connecting the positive and negative terminals of the dry cells to the bulb. The current passes out at the positive contact through the bulb and returns back via the negative contact, thus making a complete circuit.

            When the switch is moved to the ‘on’ position, a metal piece inside the case touches the positive terminal of the battery, letting the electricity flow out to the bulb. In some other torches, the battery is pushed up so that the positive contact touches the bulb itself. In a battery torch, there is a reflector which reflects back the light to long distances.

          Almost all drivers of heavy or light vehicles always carry a screw jack with them for the simple reason that it alone performs the role of several people in hours of emergency. A jack is essential equipment for raising heavy objects off the ground, usually used to remove or adjust wheels of automobiles. The screw jack is a simple machine like pulleys or levers and is used for raising large loads with a little human effort.

          A simple screw jack comprises a rod in which a square screw thread has been cut, a base plate in which this can rotate and a block (or nut) through which the screw threads runs. This block is shaped in such a manner that it hooks on to the underside of the car. When the screw is rotated by a thin rod or a tommy bar, which passes through a hole in the screw, the block is slowly raised or lowered.

          A screw thread is in fact a spiral cut made in the rod so that the screw may be regarded as a spiral inclined plane. For each complete turn of the screw it advances the same distance. This distance is the pitch of the screw, which can also be found by measuring along the length of the screw which is the distance between any two adjacent ridges of the thread. The screw of a screw jack whose diameter is 3/4 inch may have six threads per inch, so the pitch of the thread is 1/6 inch. Thus, for every one complete revolution of the tommy bar the lifting block moves 1/6 of an inch up or down.

          The velocity ratio of any machine is found by dividing the distance ‘e’ through which the effort moves by the distance ‘I’ through which the load is raised. If the tommy bar is a little more than 6 inches in length then it will trace out a circle of radius of 6 inches for each complete revolution. The circumference of this circle is about 37.7 inches so that the load is raised by 1/ 6 inch for every 37.7 inches that the effort moves. The velocity ratio of the screw jack can be calculated.

          Even if the thread of the screw is kept clean and well greased, there is still likely to be considerable friction between the screw and the lifting block as well as the base plate. Thus a load of 225 kg, which is the weight supported by one of the four wheels of an average family car, could be lifted by a 1.25 kg effort if there were no friction. But in practice the effort would be between 3.4 kg to 4.5 kg. in other words, an ideal machine which requires no energy to move its component parts, the velocity ration is equal to the mechanical advantage.

          On account of the large frictional forces between the screw and the block, the load cannot ‘unscrew’ itself and run back under its own weight.