An IT systems administrator used a grid of computers supplied by volunteers at the University of California, Los Angeles, to find the largest known prime number.

The discovery of the 13-million-digit number earns UCLA employee Edson Smith and/or his employer half of a $100,000 prize from the Electronic Frontier Foundation. The other half of the prize goes to charity.

The discovery is part of the Great Internet Mersenne Prime Search (GIMPS), a 12-year-old project that uses computers provided by volunteers to find large prime numbers. The EFF prize is for the discovery of the first prime number with more than 10 million digits.

A prime number is a whole number that can be divided only by one and itself. Mersenne prime numbers are a class of primes named after Marin Mersenne, a 17th century French monk who studied the rare numbers.

The newly found prime number is 12,978,189 digits long. If printed out, it would run 30 miles long, Smith noted.

The calculations to find the number were done using time on 75 Windows XP-based Dell desktop PCs donated by 75 students and others on the campus. Each computer ran software that GIMPS created for the task.

Very large prime numbers "are very rare and can only be discovered through computing power," Smith said. "It's really about the power of the grid."

The discovery marks the eighth prime number to be discovered at UCLA. In 1952, UCLA professor Raphael Robinson discovered five Mersenne primes that are said to be the first ones found using computers.

GIMPS is now offering up to $150,000 for the discovery of the first 100-million-digit prime number.