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170 SOUND MOTION PICTURES
highest reading, the leads of the instrument are touching battery terminals of the same polarity. If the needle deflects away from zero and below the scale, the leads on the voltmeter are touching battery terminals of the opposite polarity. If no voltmeter is available, a simple test can be made in a glass of saturated salt-water solution. Connect two wires to the terminals of the battery, scrape the ends of the wires clean, and immerse them in the salt water, holding the ends about a half inch apart. Be careful not to touch them together. If the battery is in a wellcharged condition, the formation of small bubbles at the ends of both wires will be observed. The wire around which the most bubbles collect is the negative one. This method of testing polarity can also be used in case of D. C. lighting-circuit wires. Another simple method of determining the polarity of the battery terminals is to connect two leads as explained above and to insert the bare end in the pulp of a raw potato. The ends of the wires should be stuck into the potato about a half inch apart. In a minute or so the gradual formation of a green spot in the potato will be observed around one of the wires. This wire is positive.
B. Internal Construction of Storage Batteries A storage battery consists of three individual units or cells, each capable of delivering approximately two volts of electrical pressure and a considerable current or amperage. The cells are connected in series, so that a total potential of six volts or slightly more can be obtained from the battery terminals. Each cell consists of a number of alternate brown and gray-coloured plates, which are made of a lead-alloy framework or grid, holding a hardened paste. The brown plates are positive, with a paste of peroxide of lead; the gray plates are negative, with a paste of sponge or pure lead. AH the positive plates in each