When China vowed to crack down on cryptocurrency mining early this summer, Nasdaq-listed Bit Digital Inc. ramped up efforts to get its more than 20,000 computers out of the country. The machines are the heart of the New York-based company, which makes money by plugging the high-powered computers into cheap electricity sources so they can work through mathematical problems to unlock new bitcoin. The process, called mining, has gone from something any individual with a PC could do a decade ago, to a massive industry that uses numerous computers and lots of electricity. Bit Digital and other cryptocurrency mining companies…