When static electricity collects in sufficient quantities it discharges itself in the form of a bright spark, and we shall speak of these sparks in Chapter III. Static electricity is of no use for doing useful work, such as ringing bells or driving motors, and in fact, except for scientific purposes, it is more of a nuisance than a help. It collects almost everywhere, and its power of attraction makes it very troublesome at times. In the processes of textile manufacture static electricity is produced in considerable quantities, and it makes its presence known by causing the threads to stick together in the most annoying fashion. In printing rooms too it plays pranks, making the sheets of paper stick together so that the printing presses have to be stopped. Curiously enough, static electricity has been detected in the act of interfering with the work of its twin brother, current electricity. A little while ago it was noticed that the electric incandescent lamps in a certain building were lasting only a very short time, the filaments being found broken after comparatively little use. Investigations showed that the boy was in the habit of dusting the lamp globes with a feather duster. The friction set up in this way produced charges of electricity on the glass, and this had the effect of breaking the filaments. When this method of dusting was discontinued the trouble ceased, and the lamps lasted their proper number of hours. CHAPTER II ELECTRICAL MACHINES AND THE LEYDEN JAR The amount of electricity produced by the rubbing of glass or sealing-wax rods is very small, and experimenters soon felt the need of apparatus to produce larger quantities. In 1675 the first electrical machine was made by Otto von Guericke, the inventor of the air-pump. His machine consisted of a globe of sulphur fixed on a spindle, and rotated while the hands were pressed against it to provide the necessary friction. Globes and cylinders of glass soon replaced the sulphur globe, and the friction was produced by cushions instead of by the hands. Still later, revolving plates of glass were employed. These machines worked well enough in a dry atmosphere, but were very troublesome in wet weather, and they are now almost entirely superseded by what are known as _influence_ machines.