The 13th century Venetians glass blowers are known to have produced reading stones made of solid glass that was put into hand-held, single lens-type frames made of horn or wood. These reading stones were similar to hand-held magnifying lenses of today.
The stronger the magnetic field, the larger the magnetosphere. Some 20,000 times stronger than Earth's magnetic field, Jupiter's magnetic field creates a magnetosphere so large it begins to avert the solar wind almost 3 million kilometers before it reaches Jupiter. The magnetosphere extends so far past Jupiter it sweeps the solar wind as far as the orbit of Saturn.
The higher the current, the greater the strength of the magnetic field. Like electric fields, magnetic fields are strongest close to their origin and rapidly decrease at greater distances from the source. Magnetic fields are not blocked by common materials such as the walls of buildings.