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What is an Ammeter?

What is an Ammeter?

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AmmeterAn ammeter is an instrument that measures the current in an electric circuit. The principle that governs the operation of the ammeter was discovered in 1820 by the Danish physicist Hans Christian Oersted, who found that a compass needle is deflected when it is placed near a current-carrying conductor. In 1882, Jacques Arsene d'Arsonval of France devised a moving-coil, permanent-magnet galvanometer similar to the ammeter now most commonly in use.

The moving-coil ammeter consists of a horseshoe-shaped permanent magnet, a moving coil, hairsprings, a pointer, and a scale. The movable coil of fine wire is suspended in a cylindrical air gap between the permanent-magnet poles.

This lightweight coil is pivoted on bearings so that only a small force is required to rotate it. The hairsprings, which are attached to the coil structure, establish the static, no-current position of the coil. The pointer, which is attached to the coil, moves over a calibrated scale to display the meter reading in units of amperes.

Direct current (d-c) passing through the coil produces a magnetic field that interacts with the permanent-magnet field. This interaction produces a torque that drives the coil against the hairsprings. The pointer stabilizes when the magnetic force is balanced by the spring force.

In most ammeters the pointer deflection is directly proportional to the coil current. For measuring alternating current (a-c), moving-coil ammeters have a rectifier that converts a-c to d-c. Many commercial ammeters are designed for both a-c and d-c measurements.

An ammeter can measure a wide range of currents because it has a device that allows only a portion of the current to pass through the coil. In d-c ammeters a shunt is used for this function; a-c ammeters use a current transformer.

Besides the moving-coil ammeter, there are moving-vane, electrodynamometer, and hot-wire types of ammeters. These types are used mainly for a-c measurements. Where a-c measurements must be made and it is not practical to open the circuit, clamp-on ammeters are used.

Technicians use ammeters to measure the current drain of household appliances to determine that the capacity of house lines is adequate and to troubleshoot household appliances.

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Article publié pour la première fois le 15/07/2013

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Hammers

Hammers

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HammerWhoever conceived the idea of cracking a nut with a rock unknowingly invented a tool. When a later genius tied a stick to the rock, he invented the first hammer. There have been many improvements since that humble beginning.

A claw hammer is the best choice. A well-made hammer with curved claw opposite the striking face of cast steel is a good investment.

Keep the striking surface clean and never put it away when it is wet. If the hammer head works loose, drive a wooden wedge at the top of the hammer head. Some people soak their hammers but this only swells the wooden handle temporarily and is apt to cause rust later. A sharp blow on the head will apparently tighten it, but a few moments of use will only loosen it again.

To use a hammer, grasp the handle at the end-never near the head-and be sure to strike the nail squarely and at the same angle as the direction you wish to drive the nail. Hold your hammer level with the nail-never at an angle up or down. Otherwise, the nail will bend or may come out through the wood. Use light taps until the nail is well started. Don't hurry.

The nail set is used to sink the head of the nail below the surface of the wood.

Do not use your nail hammer to work metal, or pound cold chisels, or to drive rivets. Use it only to drive nails and brads.

The metal-working hammers divide themselves into two classifications- hard-face and soft-face. One of the best general-purpose hammers is the ball peen hammer; this is often called a machinist's hammer.

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