Pure air is also an insulator of electricity, which finds its paths through the air by means of the dust particles in suspension. Moisture in the air forms a connection between the dust particles through which the electricity easily passes into the ground, but when the air is dry this medium is lacking, so that substances containing static charges are deprived of these channels of conductivity. Cold air can not hold so much moisture as warm air, so that its insulating properties are increased. It is, consequently, in cold weather when this sort of paper trouble is at its worst. These facts suggest the first steps of prevention to take against static electricity. First keep the air in the pressroom warm, and, if necessary, increase its humidity. It is also advisable to keep the paper in a warm room, for it has often been noticed that paper coming cold into a pressroom gives much trouble. The entire virtue of the so-called electric annihilators for moistening the tympan of a press comes from the moisture they contain. Ordinary glycerin, which is cheaper, will answer as well. These applications are undesirable because they cause the packing to swell, and, in consequence, detract from the adjustment of the overlays. There is a simple and not very expensive device on the market called the Thompson electrical neutralizer that has been found helpful. It is provided with a tinsel cord such as is used in decorating Christmas trees. The cord is stretched across the press so that the sheets are brushed by it as they pass to the delivery board, and are thus offered a connection whereby the static charges may escape into the ground. A second device of merit consists in a gas pipe with flames at frequent intervals over which the printed sheets pass in close proximity on their way to the delivery board. The most successful neutralizer with which the writer is personally familiar is the Chapman. By means of an alternating current of electricity, it supplies through a special apparatus alternating discharges of positive and negative electric currents against the sheets of paper as they are carried along the press. In the presence of such a current the charges on the paper become their own destroyers, as they draw out of the alternating current only the kind and quantity of electricity which is sufficient for their complete neutralization. There have been quite a number of other inventions, an account of which the writer published in _The Printing Art_, Vol. XIX, No. 1, March, 1912. All are based on one of the following principles: 1.--Making paper a conductor by moistening. 2.--Making the air a conductor by humidifying. 3.--Inducing static charges out of the paper by means of grounded wires or gas flames. 4.--Neutralizing the static charges in the sheet with charges of opposite polarity.