(overhead wire system)
The contact ran inside the n section bar, so both lateral and vertical flexibility was necessary.  A flat side-pantograph was invented in 1895 at the Baltimore & Ohio Railroad and in Germany in 1900 by Siemens & Halske. The familiar diamond-shaped roller pantograph was invented by John Q. Brown of the Key System shops for their commuter trains which ran between San Francisco and the East Bay section of the San Francisco Bay Area in California. They appear in photographs of the first day of service 26 October 1903 For many decades thereafter, the same diamond shape was used by electric rail systems around the world and remains in use by some today.
The pantograph was an improvement on the simple trolley pole which prevailed up to that time primarily because it allowed an electric rail vehicle to travel at higher speeds without losing contact with the catenary.
The (asymmetrical) 'Z'-shaped pantograph of the electrical pickup on the Berlin Straßenbahn. This pantograph uses a single-arm design. 
The (asymmetrical) 'Z'-shaped pantograph of the Desiro Class 360/2 EMU  on the Suvarnabhumi Airport Rail LinkThe most common type of pantograph today is the so called half-pantograph (sometimes 'Z'-shaped), which has evolved to provide a more compact and responsive single-arm design at high speeds as trains get faster. The half-pantograph can be seen in use on everything from very fast trains (such as the TGV) to low-speed urban tram systems. The design operates with equal efficiency in either direction of motion, as demonstrated by the Swiss and Austrian railways whose newest high performance locomotives, the Re 460 and Taurus respectively, operate with them set in opposite directions.
Pantographs easily adapt to various heights  of the overhead wires by partly folding. The tram line pictured here runs in Vienna. The electric transmission system for modern electric rail systems consists of an upper weight carrying wire (known as a catenary) from which is suspended a contact wire. The pantograph is spring loaded and pushes a contact shoe up against the contact wire to draw the electricity needed to run the train. The steel rails on the tracks act as the electrical return. As the train moves, the contact shoe slides along the wire and can set up acoustical standing waves in the wires which break the contact and degrade current collection. This means that on some systems adjacent pantographs are not permitted.
Pantographs are the successor technology to trolley poles, which were widely used on early streetcar systems. Trolley pole are still used by trolleybuses, whose freedom of movement and need for a two-wire circuit makes pantographs impractical, and some streetcar networks, such as the Toronto Streetcar System, which have frequent turns sharp enough to require additional freedom of movement in their current collection to ensure unbroken contact.
Pantographs with overhead wires are now the dominant form of current collection for modern electric trains because, although more expensive and fragile than a third-rail system, they allow the use of higher voltages.
Pantographs are typically operated by compressed air from the vehicle's braking system, either to raise the unit and hold it against the conductor or, when springs are used to effect the extension, to lower it. As a precaution against loss of pressure in the second case, the arm is held in the down position by a catch. For high-voltage systems, the same air supply is used to "blow out" the electric arc when roof-mounted circuit breakers are used.
Single- and double-arm pantographs
High-performance pantograph for measurements on the ICE SPantographs may have either a single or a double arm.  Double arm pantographs are usually heavier, requiring more power to raise and lower, but may also be more fault tolerant. For example, "... [New Jersey Transit] encountered numerous wire downings on the Northeast Corridor Branch (New York City - Trenton, NJ) before they decided to replace the pantographs on Arrow-III trains with a more forgiving dual arm design, possibly in 1991..."
On railways of the former USSR, the most widely used pantographs are those with a double arm ("made of two rhombs"), but since the late 1990s there have been some single-arm pantographs on Russian railways. Some streetcars use double-arm pantographs, among them the Russian KTM-5, KTM-8, LVS-86 and many other Russian-made trams, as well as some Euro-PCC trams in Belgium. American streetcars use either trolley poles or single-arm pantographs.
systems and overhead lines
Symmetrical, diamond shaped pantographs on trams in Prague  .Most rapid transit systems are powered by a third rail, but some use pantographs, particularly ones that involve extensive above-ground running. Hybrid metro-tram or 'pre-metro' lines whose routes include tracks on city streets or in other publicly-accessible areas, such as the MBTA Green Line, must of course use overhead wire, since a third rail would normally present too great a risk of electrocution.
The only current exception to this is the new Bordeaux tram system that uses an underground system called alimentation par sol, which only applies power to segments of track that are completely covered by the tram. This system is used in the historic centre of Bordeaux where an overhead wire system would cause a visual intrusion.
Overhead pantographs are sometimes used as alternatives to third rails because third rails can ice over in certain winter weather conditions. The MBTA Blue Line or the Wonderland Line uses pantograph power for all of its surface route. The entire Metro system of Barcelona, Spain, uses overhead wiring and pantographs.
Until 2010 the Oslo metro line 1 changed from third rail to overhead line power at Frøen station. Due to the many level crossings, it was deemed difficult to install a third rail on the rest of the older line 1 tracks .
See also Wikimedia Commons has media related to: Pantograph
Railway electrification system http://www.youtube.com/watch?v= WYleLB87g_o&NR=1
click photos once to enlarge, twice for maximum enlargement.