Page:LA2-NSRW-2-0086.jpg





superstructures of astronomy, physics and mechanical engineering.

For the history of the subject see Mach’s Science of Mechanics, translated by McCormack; for the laws of motion see Tait’s Newton’s Laws of Motion; for a treatment of rotation see Worthington’s Dynamics of Rotation.

Dynamite, from a Greek word meaning strength, is produced by mixing with a kind of earth, known under its German name of keiselguhr.  Nitroglycerine was discovered in 1846, but it was not till 20 years after that the experiments of Alfred Nobel, who combined it with the earth just mentioned, made it of practical importance. Dynamite has a reddish-brown color, and is about one part earth to three parts nitroglycerine. It burns with a yellowish flame and, in small quantities, without danger. The time of an explosion of a dynamite-cartridge is about the 24,000th part of a second. Dynamite is much used for breaking up bowlders, and is used under water as well as on land, the water causing the loss of only six per cent. of its power. For quarrying purposes gunpowder is generally used, as dynamite breaks up rock too much. Dynamite is also used in explosive shells that may be fired from guns of special construction, a dynamite gun having been invented in 1883. Unfortunately, it has been also employed for the destruction of life and property.

Dynamo. See.

Dynamo-Electric Machine or, simply, dynamo, a machine for generating an electric current in a conductor by moving the conductor in the magnetic field of a magnet or system of magnets. The motion is relative, that is, either the conductor or the magnet may be the moving part. All dynamos are based upon the fundamental discovery made by Michael Faraday in 1831. Faraday discovered that, if a wire forming a part of a circuit is moved across the space (called a magnetic field) in front of the pole of a magnet, an electric current will be set up in the wire and circuit during time of motion. The modern dynamo consists of two essential parts: (1) an electromagnet or system of electro-magnets called the field-magnets and (2) a system of copper wires, usually wound on a laminated iron core. This system of conductors is called the armature, and is the part in which the current is generated. In most varieties of dynamos the armature revolves in the magnetic field of the field-magnets, but in many large alternating-current dynamos the field-magnets form the revolving part.

Dynamos are divided into two general lands: direct current (D. C.) and alternating-current dynamos (A. C.) or simply alternators. The current generated in the circuit of a D. C. dynamo is constant in direction and magnitude. The current produced by an alternator reverses direction at regular intervals. All commercial D. C. dynamos have a commutator. This is a metal cylinder built up of a number of insulated sectors or bars, which revolves with the shaft on the armature. Fixed conducting-brushes rest against this commutator and make the connections with the external circuit. The armature-coils are connected with the commutator-bars, so that the connections with the external circuit are reversed as often as the current in the armature is reversed. In this way the current in the external circuit is kept constant in direction. The collecting device for a simple A. C. dynamo consists of two insulated copper-rings on which the brushes rest. The current in the external circuit is thus reversed as often as the current in the armature-coil is reversed. The common number of alternations or reversals (often called cycles) in commercial alternators is either 125 or 60, but as low as 25 are used in the Niagara Falls electric plant.

Alternators are often built to generate two or more alternating currents, which differ in “phase.” Such machines are called two- or three-phase alternators. Thus a two-phase machine is one which generates two separate alternating currents, the two currents differing in that the first reaches its maximum when the second is at zero, and when the second reaches a maximum the first is passing through its zero, and so on. Apparatus generating and using two or more phases is called polyphase apparatus.

The use of alternating currents has increased largely within the last 15 years, because of the inventions of transformers and of the induction-motor. The polyphase alternating-current system is the only one commercially feasible for long-distance transmission of electric energy

Image: SIMPLE DIRECT CURRENT DYNAMO

Image: SIMPLE ALTERNATING CURRENT DYNAMO