While building the bridge the workers used new materials.

To study geometry means to train logical thinking.

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ATOM AND ELECTRON

Present atomic theories, based on the atom model, place the mass and the positive charge of an atom in a central nucleus, surrounded by moving electrons. The nucleus is composed of neutrons and protons. Neutrons have a mass approximately one-sixteenth that of the mass of the oxygen atom and have zero electrical charge. The proton has a mass slightly lighter than that of the neutron and approximately equal to the mass of the hydrogen H.

The chemical elements are differentiated by varying numbers of protons and neutrons in the nucleus, numbers of protons being equal to the atomic number, hydrogen having one, helium two, and so on.

The electron is the smallest known indivisible unit of electric charge. Early experimenters succeeded in the measuring the ratio of the charge of the electron to mass, and in 1910 the charge was first measured, thus determining these properties of the electron.

The electron appears to have a dual personality, at times being best thought of as a particle, and in other experiments seeming to require wave properties for a satisfactory explanation of the phenomena.

Electric current is due to the movement of electric charges, frequently referred to as electrons. However, long before the existence of the electron was known, it was customary to speak of electric current as due to movement of positive charges from positive to negative in a metallic circuit external to the source.

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