As we look about us we find electricity moving the cars on which we ride and producing the light by which we see at night, and we naturally ask, "What is electricity?" That question cannot, as yet, be answered definitely. Electricity is no doubt a form of energy having properties of its own, but obeying laws corresponding quite closely to those governing the motion of water. A great many explanations can be offered by comparing the action of electricity with water. For example, electricity flows through a wire in much the same way as water flows through a pipe. From their likeness it has become popular to speak of electricity as "juice."

Fig. 70.   Mariner's Compass.

Fig. 70. - Mariner's Compass.

Since electricity is, in a sense, considered a fluid, its flow is called a current, and any substance, such as copper wire, through which it flows is called a conductor. All metals, salts, and solutions, living vegetable substances, and water, are conductors of electricity. There are some bodies, however, such as glass and rubber, that offer a resistance so great as to prevent the passage of electricity. Most vegetable substances in a dry state, such as shellac, resin, rubber, paper, and cotton are in this group. Other non-conductors are wood, sulphur, glass, mica, silk, porcelain, and oil. The path through which a current passes is called a circuit. When a path is continuous it is called a closed circuit, but when there is a break it is called an open circuit.