What law describes the fact that on average computers have doubled in capacity every 18 to 24 months since 1900?

Moore's Law explains this effect from about 1964 to the present, due to the introduction of monolithic silicon integrated circuits.

However prior to 1964 there was no such effect, each component for a computer had to be individually assembled and packaged whether it was a transistor, vacuum tube, etc.

Prior to 1942 there were no electronic computers at all, calculations were done by hand or with the aid of mechanical or electromechanical calculating machines. The most automatic of these were punchcard unit record equipment programmed using plugboards, and prior to about 1949 the most advanced of these could only do multiplication so you had to take the output deck to another machine that could only do addition/subtraction to add up the products.

So from 1900 to 1964 computing capacity did not follow Moore's Law at all!