Static electricity may easily be drawn off and bottled up in what is called a Leyden jar. This is a glass jar (Fig. 82) three-quarters of the surface of which is coated inside and outside with tin-foil. A brass rod with a knob at the end goes through the cork and into the jar until it touches the inside coating of tin-foil. If the knob of this jar be held about half an inch from the conductor of an electrical machine, sparks will pass for some time from the conductor to the knob of the jar and will then cease. The jar is then said to be charged, that is, the coating on its inside is as full of electricity as it will hold. The jar can be charged only when the outside is connected with the earth; if the outside be insulated, no electricity can be collected in it. It is enough to hold the outside of the jar in the hand, as in this way it is connected with the earth through the body. The positive charge from the conductor then passes into the inside coating of the jar.