Magnetic wire sound recordings made in the 1890’s are still good. This fact shows that magnetic wire may be a more permanent medium for storing information than is paper. Stray magnetic forces are likely to have no harmful effect on information stored on magnetic wire, for these forces would not be strong enough or detailed enough to change greatly the difference between the magnetized spot and its neighboring neutral area. A reel of magnetic wire a mile long and ³/₁₀₀₀ of an inch thick costs about $5. At 80 magnetized spots to the inch, a mile of wire can store about 5 million units of information. Hence, the cost of storing one unit of information is about ¹/₁₀₀₀₀ of a cent. The time needed for changing a magnetized spot from 1 to 0 or from 0 to 1 is about ¹/₁₀₀₀₀ of a second. Magnetic Tape There is, however, a storage device that may be even more useful, and this is _magnetic tape_ (see Fig. 1). The usual size of such tape is ¼ inch wide and 2 or 3 thousandths of an inch thick. Magnetic tape may be made of plastic with magnetic powder all through it, or it may be of paper coated with magnetic powder, or it may be of stainless steel or a magnetic alloy, or it may be of brass or a nonmagnetic alloy coated with a magnetic plating. Magnetic tape has the added advantage that from 4 to 20 channels across the tape can be filled with magnetized spots, and the cost then becomes about ¹/₁₀₀₀₀₀ of a cent per spot. It seems possible that 1000 units of information can be stored in a quarter of a square inch of magnetic tape. This means that more than 1 million units of information can be stored in a cubic inch of space filled with magnetic tape, and about 2 billion units of information in a cubic foot, except that some of the space should be allotted to the reels and other equipment that hold the tape (see Fig. 2). This is closer packing than printed information in the telephone book, and yet with magnetic tape we can get to the information automatically. [Illustration: FIG. 1. Magnetic tape.] [Illustration: FIG. 2. Tape reels.] Think of the enormous files in libraries, government, and business. Think of the problems of space and cost and access which these files imply. We can then see that this new development may well be of extraordinary importance. Mercury Tanks [Illustration: FIG. 3. Mercury tank.]