In a paper published in the ‘Philosophical Magazine’ of May, 1913, an account is given of some experiments which show that the raising of a cloud of dust is accompanied by the production of a large amount of electricity. The dust was usually blown away from the surface of a Hat piece of material or from the end of a tube. Large quantities of electricity were produced by this means and the sign of the charge seemed to depend upon the nature of the material used to form the dust cloud. Basic bodies generally acquired a negative charge, and acidic bodies a positive charge. It might be suggested that the origin of the charges lay in the friction of the particles of material against the surface from which they were blown, but this, however, is not altogether the case, as the following experiment will show. An insulated brass tube, 25 cm. in length and 1·5 cm. in diameter, was connected to an electroscope. Tubes of different materials and of a slightly smaller diameter were provided to fit inside it so that the nature of the surface from which the dust was blown could be varied. Under these circumstances it might be expected that the charge carried away by the dust, and also that retained by the tube, would vary with the nature of the surface, but this was not the case, for with the same substance for creating the dust, the electrification was always the same no matter what was the nature of the surface from which it was blown. For example, sand always furnished a positive charge and red lead a negative one, and other materials gave charges of the signs given in the paper referred to. Again, the nature of the surface against which the dust was projected was also without influence upon the charge acquired. Clouds of dust were raised by blowing a current of air through the dust contained in a small wash-bottle, and the cloud projected against an insulated plate connected to an electroscope. The charge gained by the plate was of the same sign whether the plate was rough or smooth, or coated with different materials, so that it appears evident that the charge is actually produced upon the dust cloud itself. It has been shown that the charge upon the dust is apparently accompanied by one of opposite sign upon the air, and a convenient arrangement for showing the presence of these two charges is given in fig. 1. The dust raised by blowing air through W is projected against the wire gauze screen G and the greater portion is caught, but the air accompanying the dust is carried on. At some distance (50-100 cm.) behind the gauze a small radium-coated plate E served to collect any charge present upon the air. Electroscopes E
2
and E
1
indicate the charges acquired by G and R. A few typical cases are given in Table I.
On the origin of negative target currents during laser ablation of polyethylene