How does a ball pen and a gel pen work?

Surface tension of the ink forms a thin film of the ink on the ball. As the pen moves over the paper this film is deposited on it. The engineering is in allowing the ball to rotate,not get pushed into the reservoir and have just enough clearance so that the ink does not leak. Poorly made pens can leak ink around the ball as many a spoilt shirt can bear witness. The problem is that the pen has to work over a wide range of ambient temperatures. The formulation of the gel takes care of this.
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I remember in 1948 when the ball points were first sold in India at a huge cost. The company came house to house to demonstrate. They also gave a free book,'I chose freedom' by Victor Kravchenko with each pen. The pen, which was a novelty and not too good cost a present day equivalent of Rs.250.