Amateurs frequently desire to learn the polarity of their primary or secondary battery terminals. There are several contrivances constructed of glass and depending upon the coloring of the solution contained therein for a polarity indicator. A simpler method is to cut sheets of white blotting paper into small squares 2 by 2, and saturate the squares with iodine of potassium and allow them to dry. When a test is to be made, moisten a piece of the paper with water and press the battery leads down upon the wet surface about 1/2 an inch apart. The positive end will turn a pronounced brown, while the negative end does not change.

A strip of glass cut one inch by four, coated with shellac and sprinkled with fine brass or other metal filings, will form a magnificent display when introduced between the balls of a secondary spark, especially at night. A pretty design or a person's name may be traced in minute sparks by this method.