Illustration showing the difference between a direct current and an alternating current.

A map of Africa showing the continent prior to the Berlin Conference of 1885, when the most powerful countries in Europe at the time convened to make their territorial claims on Africa and establish their colonial borders at the start of...

A map from 1868 of North America, Central America, and the Caribbean showing the physical features of the region, vegetation types, ocean currents, temperatures, principal products and mineral deposits. Mountain chains are represented by...

A map from 1868 of South America showing the physical features of the region, vegetation types, ocean currents, temperatures, principal products and mineral deposits. Mountain chains are represented by lines of varying thickness, with...

A map from 1868 of Europe, showing the physical features, climate, and commercial resources of the region. Mountain chains are represented by lines of varying thickness, with the broader the line, the greater the elevation. The highest...

A map showing area precipitation and the direction and effects of wind currents on the African continent, including the desert regions in the trade wind and horse latitude belts, and the region of heavy rainfall in the belt of calms.

A map from 1910 of Africa showing area precipitation and the direction and effects of wind currents on the African continent, including the desert regions in the trade wind and horse latitude belts, and the region of heavy rainfall in...

A map from 1868 of Asia showing physical features and natural resources of the region. The map shows political boundaries of the time, with capitals, major cities, and ports. The map shows mountain systems with black lines of variable...

A map of Asia from 1872, showing physical features including mountain systems, deserts, lakes, rivers with direction of flow, coastal features, and ocean currents, political boundaries and foreign possessions at the time, and commercial...

A map from 1901 of Europe, showing the physical features of the region. This map is color-coded to show general elevations from sea level to over 6,500 feet, mountain systems, drainage divides, lakes, rivers, coastal features, and the...

A map from 1872 of the West Indies and Central America. The map shows political boundaries at the time, capitals, major cities, and ports, railroads, main steamship routes, rivers, terrain and coastal features, and banks. The map shows...

A map from 1901 of South America showing the physical features of the region. This map is color-coded to show general elevations from sea level to over 6,500 feet, mountain systems, drainage divides, lakes, rivers, coastal features, and...