If we compare ballpoint pen with fountain pen it is cheaper and less messy.
As the name suggests we use ball in the ball point pen which is made up of steel, brass or tungsten carbide. The ball always remains rotating on the tip to disperse the ink while we are writing. Ball is located between the ink reservoir and paper and acts as the medium between the paper and the ink. It is placed in such a way that it has enough to roll while writing. Due to the forces of gravity ink moves downwards from the reservoir to the ball and when we write on paper ball rotates and the ink is dispersed onto the paper. It is this rolling mechanism that permits the ink to flow to the top. Reservoir is also sealed from the air which makes the ink not to dry inside the reservoir.
Width of the line depends on the size of ball for e.g.:- 0.5mm of ball will produce a line of width 0.5mm and 0.7mm of ball will produce a line of width 0.7mm. It can be 0.1mm wide which is known as ultra fine pens.
Before the ball point was invented the other pens like fountain pen used watery dark ink that moves to the top by capillary action but it faced some problems like the ink dries slowly if it dries in the pen than it requires meticulous cleaning. So ball point pen is considered much better as it uses fast drying ink which does not dry inside the reservoir.
Erasable ball point pen uses ink that is made up of liquid rubber cement instead of dyes and oils and it can be easily erased after writing. Normally up to ten hours it can be erased after that it will become hard and non-erasable.
Some manufactures have made ball point pens combined with lighted diodes in which the words appear shinning for 300 hours.
Ball point pen works on the gravity force and the opening on the top which permits the air to replace the space of used ink. While the pens used in space and underwater are pressurized ball pens and the ink used in it is special viscoelastic ink