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Moving Coil Meters for Electrical Measurements of Current & Voltage

Electrical measurements of current or voltage are usually made by a moving coil meter. The meter construction is the same for each but its arrangement in the circuit is different.

A moving coil meter consists of a coil wound on a soft iron cylinder which is pivoted and free to rotate (Figure below). Two hair springs are used, one above and one below, to provide a restraining force and also to conduct the current to the coil. The moving coil assembly is surrounded by a permanent magnet which produces a radial magnetic field. Current passed through the coil will result in a force which moves the coil against the spring force to a position which, by a pointer on a scale, will read current or voltage.

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The instrument is directional and must therefore be correctly connected in the circuit. As a result of the directional nature of alternating current it cannot be measured directly with this instrument, but the use of a rectifying circuit will overcome this problem.

Moving coil meter
Fig:Moving coil meter

Ships instruments:

Oxygen analyser working principle
The measuring of oxygen content in an atmosphere is important, particularly when entering enclosed spaces. Also inert gas systems use exhaust gases which must be monitored to ensure that their oxygen content is below 5%. One type of instrument used to measure oxygen content utilises the fact that oxygen is attracted by a magnetic field, that is, it is paramagnetic....

The measurement of pressure by manometer or a Barometer
A U-tube manometer - One end is connected to the pressure source; the other is open to atmosphere. The liquid in the tube may be water or mercury and it will be positioned as shown.....

Liquid-in-glass thermometer
Various liquids are used in this type of instrument, depending upon the temperature range, e.g. mercury -35C to +350C, alcohol -80C to 4-70C. An increase in temperature causes the liquid to rise up the narrow glass stem and the reading is taken from a scale on the glass . High-temperature-measuring mercury liquid thermometers will have the space above the mercury filled with nitrogen under pressure.....

Level measurement by Pneumatic gauge
This is a device which uses a mercury manometer in conjunction with a hemispherical bell and piping to measure tank level. The arrangement is shown in Figure. A hemispherical bell is fitted near the bottom of the tank and connected by small bore piping to the mercury manometer. A selector cock enables one manometer to be connected to a number of tanks, usually a pair....

Salinometer -How to determine water purity ?
Water purity, in terms of the absence of salts, is essential where it is to be used as boiler feed. Pure water has a high resistance to the flow of electricity whereas salt water has a high electrical conductivity. A measure of conductivity, in Siemens, is a measure of purity.....

Oil in water monitor
Current regulations with respect to the discharge of oily water from ships set limits of concentration 15 parts per million. A monitor is required in order to measure these values and provide both continuous records and an alarm where the permitted level is exceeded.....

Viscosity control of marine fuels
Viscosity control of fuels is essential if correct atomisation and combustion is to take place. Increasing the temperature of a fuel will reduce its viscosity, and vice-versa. As a result of the varying properties of marine fuels, often within one tank, actual viscosity must be continuously measured and then corrected by temperature adjustment. ....

Flow measurement
Flow measurement can be quantity measurement, where the amount of liquid which has passed in a particular time is given, or a flow velocity which, when multiplied by the pipe area, will give a rate of flow.....

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