The December 1940 issue of Popular Mechanics showed these plans for making a resistor out of the carbon from an old flashlight battery. After removing the rod from the battery, it was boiled in a strong soda solution and dried. A grid cap, normally used to snap on to the terminal on top of a tube, was the perfect size to clip on to each end. A single carbon rod was about 50 ohms, and two of them could be joined together to form a center-tapped 100 ohm resistor.