avoided by the use of the ripping line on landing. The escape of gas then takes place through so large an opening that the velocity with which the gas passes the edges of the orifice is small, and the friction not sufficient to produce a sparking charge.
The friction between a pulley and its belt may in a dry atmosphere produce so strong a charge in the belt that sparks may be drawn from it. Such sparks are quite harmless to any person struck by them, but they may become a source of danger if inflammable substances are near. Thus in paper-making machinery, where the band of paper passes at high speed over hot metal rolls, it may become electrified to the extent of sparking and igniting itself. To avoid this danger it is necessary to fix spiked combs, which draw off the charge as soon as generated. In all these cases the electricity produced by friction is only an inconvenient by-product of some other operation; but if we wish to produce electricity for experimental work we may use special appliances based on the principle of electrification by friction. These are called "frictional machines." In substance they are nothing more than elaborations of the primitive glass rod and leather
