I haven't tried this, but the idea would work approximately as follows. The calculator is switched on and placed inside a microwave oven. Close the oven door, switch the oven on, then immediately switch it off.
When you remove the calculator, it should register numbers on its display. The numbers are random, to be sure, but it is for sure a novel way to get numbers onto a calculator.
(If you leave the calculator inside the microwave oven too long, the calculator will be destroyed.)
Interlocutor, No. 20, 1977, p. 76