This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
For testing dry batteries or any low-voltage current, take an ordinary thermometer and wind around the mercury bulb enough wire to make about 10 ohms resistance. This will make a good tester. A dry cell of about 2 volts attached to the ends of the wire should generate enough heat to expand the mercury about four degrees in one-half minute.
This tester is not as fast as a voltmeter, nor has it as wide a range, but it is reasonably accurate, and by using a battery of known voltage, the winding can be increased or diminished to allow the mercury to expand as many degrees as desired per volt. - Contributed by E. H. Kimbrough, Bart-lett, Kan.