How does the speed (of vehicles) measuring device used by the traffic work ?

Chronometers are used to determine the speed of the vehicles with a hand held stopwatch. No radar, no loop just a plain and simple digital stop watch, and two points of a known distance apart. Start and stop switches are to be manually actuated when the vehicle crosses the points.

The speed will be displayed instantly. The second method is by using a Piezo Sensors, this method uses set of 3 rubber strips with a specific distance apart are build across the road or buried in the road. The time between comparisons is measured to give the resulting speed of the vehicle. If the speed exceeds the camera associated with the system will photograph the vehicle.

In another method two beams of invisible infrared beams are emitted across the given traffic lane. The elector optic head directs the beams to two retro-reflectors mounted in the road surface.

Every vehicle passing between the electro optic head and the reflectors breaks beams and triggers the unit computer to measure the speed. If a vehicle exceeds speed limit, the high resolution camera photographs the front or rear of the vehicle including its license plate.

In another method, infra red diodes are pulsed at certain frequencies and to create radar beams. This beam is emitted via parabolic reflector at 15 degree spread and directed towards traffic. The emitters are shielded to prevent external interference.

They normally use K band in a frequency 24 GHZ. The associated computer will measure speed and triggers camera to capture the image of erring vehicles.

What is the black box (in planes) made of ? Normally wat does it do ?

Black boxes which usually hold the clues to a plane's crash, are built strong enough to survive bomb blasts, violent impacts and intense fires. They measure approximately 4 by 6 by8 inches and weighs about 30 kg and are kept in a case which can withstand 30 minutes of 1100 degrees C at 50,000 (Btu's) British Thermal Units per square foot per hour.

They are actually orange in color and a very high quality heat resistant paint is used, so that they can be spotted easily even at dimly lit crash sites. They are always placed in the tail end of the flight.

The black box records voice and flight data. The cockpit voice recorder continuously collects data and retains a record of the most recent 30  minutes, the conversation between the pilot and the Ground Control Room.

The flight data recorder keeps a record of the most recent 25 hours of a plane's operating data, including altitude, air speed, vertical acceleration and heading.

Flight data are recorded either by magnetic tape or in solid-state memory. Modern data recorders can long more than 200 different parameters and transmit data either 64 times or 128 times a second. Thus when recovered from the airplane crash sites the black box helps to reconstruct a clear picture of the crash.

How does a cell phone work?

In normal telephone exchange system,a pair of wires extend each telephone instrument to the telephone exchange.

Normally,copper conductors are used to give loop to indicate the exchange that the telephone handset had been lifted to making a call or receiving a call.In advanced country s, Optical Fiber is also used for extending the loop.

When you make a call to your friend who is connected to another exchange,the digits are send to the  exchange to which you are connected,in the form of pulses (break and make of loop) or frequencies.

The originating exchange analysis these digits and establishes a path to the terminating exchange where your friend is connected .Over this path,signals(some of the digits dialed by you and other technical information)are send in form of bits or frequencies.

The terminating exchange analyzes and marks your friend's line and if it is free, a ring is sent to him and ring back tone is sent to you. When the call is answered, metering takes place.

In the cell phone system, there is no permanent wire connection between the instrument and the mobile telephone exchange. The connection between the instrument and the mobile exchange is established via intermediate station called base station (BS) where a transmitting tower is erected. The connectivity between the mobile exchange and the BS may be the wire and between BS and cell phone is radio channel.

The connection between the mobile exchange and another mobile exchange and public telephone exchange is by wire.

Cell phone are technically called mobile station (MS) and its telephone exchange is called Mobile Switching Centre (MSC). The radio channel for both-way voice and control channel for controlling are established between BS and MS  when required only.

A town or an area is divided into smaller areas called cells.At the center of the cell,there exist a Base Station(BS).When the MS moves inside the cell,the signals including voice will be strong.When you use a cellphone to talk to your friend with conventional phone,you will be connected to the BS over a both way radio channel.From BS to MSC over a copper cable or Optic Fibre cable,from MSC to another exchange and further to your friend over a copper cable.

As you are moving and when you enter into another cell,old BS reports to MSC that you have left the cell and your signals are weak.Immediately ,the MSC order all other Base Stations to search for a newcomer (as far as other cells are concerned ,you are newcomer).

The BS which is very near to you reports to MSC that it has found a new comer with strong signals.The MSC orders the old BS to hand off (you) to new BS. Handing off is done in less than 400 milliseconds. But you do not feel any disturbance in your conversation. You may cross many cells during your long conversation without knowing that so many hands off had taken place. Each cell phone is associated with a unique identity stored in subscriber identity module (SIM). This identity is sent to MSC through BS when "send" button is pressed. 

The MSC analyzes this and validates as to whether cell phone is authorized to make a call. If yes , the call will be put through. When there is any incoming call to the MS, first it is received by the MSC. MSC analyzes the received information that contains the MS number also. This number is paged in all cells. If the MS is kept power on, a page response is received on the control channel from the MS.A voice channel is allotted by MSC. BS and MS are ordered to tune to that channel. Once tuned through connection information is returned to the caller who gets ring-back tone. Ringing is initiated in the MS. When the MS answers, conversation can start and metering starts in MSC.