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.
Lines in a cable or the ends of connections at a distance from the battery must be tested to determine the polarity. Where a large amount of this work is to be done, as in automobile and motorboat repairing, it is necessary to have an indicator to save time. A cheap indicator for this purpose can be made of a 6-in. test tube having its ends sealed and inclosing a saturated solution of ammonium chloride (sal ammoniac) and water. The sealed ends are made by inserting a piece of wire through a cork and, after forcing this tightly into the end of the test tube, covering it with sealing wax.
To use, connect the terminals to the battery lines, and the end of the wire in the solution giving off bubbles is the negative wire. - Contributed by H. S. Parker, Brooklyn, N. Y.
Ill: A Simple Pocket Indicator for Finding the Negative Wire in Battery Cable Lines