An electric filter has recently been exhibited at Philadelphia.

The apparatus was placed in a long trough like a wooden tank, through which ran the water to be purified. In it were suspended plates of aluminum, parallel with the sides of the trough—three sets of them, one set of plates at each end, and one in the centre These plates were thirty-six inches long, and six inches wide, and were arranged like the plates of an electric battery, alternately positive and negative. The positive wire of a dynamo ran on one side of the trough, and the negative on the other, and the three sets of plates were connected between them, in what is known as “ multiple” connection. In other words the tank, or trough, resembled an electro plating tank. The dynamo current was of low voltage and passed a current of about twenty amperes through the water. The water was introduced at one end of the tank and flowed between the plates and out of the other end. As the water passed out, a blast of combined air and ozone was forced through it. The theory of this method is that the electric current in passing between the plates, which are positive and negative, through the water, decomposes a portion of the water, the gases combining with aluminum to form the hydrated oxide of alumina. Phis is the same result as is produced by an alum filter; and it has recenly been demonstrated that this freshly precipitated hydrate oxide of alumina can be better prepared from metallie aluminum than from the sulphate of alumina, as heretofore. The amount of decomposition depends much upon the amount of impurity in the water. 'I his gelatinous hydrate of alumina, as it falls to the bottom, carries down the suspended impurities to the bottom of the tank,and the ascending gas of the decomposed water carries up the lighter impurities to the surface. The floating scum is caught by a scraper, after each set of plates, which leads it out of the tank; the precipitated deposit is removed by dumping it from the hinged bottom. As the water flows from the first to the second set of plates, it be comes clearer, and at the third set has become comparatively clear. The water used was like the Schuylkill in its flood stage. The electrical apparatus consisted of a generator of direct current for the tank. The ozone producer was of a more complicated character. It was much like an X ray outfit. There was first an alternating current generator of 110 volts and three amperes for exciting the primary of a large induction coil, which was insulated with oil. In the secondary of this coil was a large battery of Leyden jars—the terminals of the secondary being connected to the primary of a second induction coil, immersed in oil, which is said to transform in the ratio of 500 to 1. This combination produces a powerful spark at the terminals of its secondary winding, which was discharged through a glass tube a distance of three feet, nine inches. Through this same tube an air pump, run by power, forced condensed air, a portion of which was converted into ozone and this current of ozone was forced through the water as it left the trough. Samples of the purified water were taken by the chemist and bacteriologist for tests as to its purity and freedom from bacteria. Such a plant would filter 2,000,000 gallons daily and cost $75,000.

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