The magnetic fields generated by currents and calculated from Ampere's Law or the Biot-Savart Law are characterized by the magnetic field B measured in Tesla. But when the generated fields pass through magnetic materials which themselves contribute internal magnetic fields, ambiguities can arise about what part of the field comes from the external currents and what comes from the material itself. It has been common practice to define another magnetic field quantity, usually called the "magnetic field strength" designated by H. It can be defined by the relationship
Magnetic fields can be produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property , their spin .   In special relativity , electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic tensor ; the split of this tensor into electric and magnetic fields depends on the relative velocity of the observer and charge. In quantum physics , the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons .
Top 5 European cities to visit in autumn.
If you want to see the Eiffel Tower, gain strength in Budapest Baths, wander through Old Riga or make a wish on the Charles Bridge in Prague, here are five ideal destinations for weekend vacations in November and December.
Prague, Czech Republic.
A famous journalist Peter Vail once said that Prague gives the idea of the whole Europe. And it is true. After all, Prague remains an amazing city incorporating the best features of this part of the world.
Tourists come here to admire red shingles, listen to the legends about the Golem and alchemists, wander through the tangle of medieval alleys, drop into St. Vitus Cathedral and make a wish on the Charles Bridge. A mandatory travel program for Prague also includes walks through the Old Town Square, acquaintance with the ghosts of Visegrad and, of course, tasting Czech beer in local cafes.
Paris has been among the top three in the list of the most popular tourist destinations in the world for many decades. The splendid Champs Elys?es, famous Gioconda in the Louvre, magnificent Notre-Dame Cathedral and beautiful views from the height of the Eiffel Tower are its main trump cards.
The French capital is also loved for the bustling life of its boulevards, the richness of the Orsay Museum, the pretentiousness of the Pompidou Centre, the business district of La Defense and its tiny cafes in Montmartre.
A short weekend in the capital of Hungary is a great opportunity to combine business with pleasure. Here you can see the most beautiful parliament building in Europe, walk along the so-called Hungarian Champs Elysees, i.e. Andrassy Avenue, enjoy a voyage along the Danube, admire the famous Chain Bridge from the water, and, what is most important, visit Budapest Baths.
There are more than 80 thermal springs on the territory of Budapest. This is the only capital in the world, which has the status of a health resort. Visit Szechenyi Baths, which are considered to be the biggest bath complex in Europe, admire the columns and marble of Gellert Baths, and swim in panoramic Rudas Thermal Baths.
It is impossible to describe Berlin in brief. This is a cosmopolitan city, the streets of which belong to tourists. This is a park, where you can fearlessly lie on the lawns. This is a museum, and even a month will not be enough to visit all its exhibitions. You should go to the Berlin Zoo, which is considered to be one of the largest and most beautiful zoos in the world, get into the Reichstag dome, and, of course, walk around the Museum Island to see the Nefertiti Bust, the Pergamon Altar and other treasures of antiquity.
Also, you have an opportunity to visit the nightclubs of Kreuzberg and Friedrichshain, walk along the Tiergarten and shop at the famous Ku`damm.
Despite its small size, you can stay in the capital of Latvia for several weeks. You will probably come here for the sake of the salty smell of the sea, cozy cafes, where you can taste delicious coffee, a measured rhythm of life and the picturesque Old Town. In Riga, you can spend hours looking at pretty mansions in the streets of Alberta and Elizabetes, walk around Jauniela, admire the broken lines of tiled roofs from the viewing platform of St. Peter`s Church, and enjoy organ concerts in the Dome Cathedral.
Dedicate a couple of hours to a trip to Jurmala. The local pines, sea sunsets and sand dunes relieve the autumn spleen better than medicines do.