AMMETER

An ammeter is an instrument used to measure currents in electric circuits directly in ampere (A). The instrument measuring currents of the order of milliampere  (mA) is called milliammeter.An ideal ammeter has zero resistance.Ammeter is essentially a galvanometer which is inserted in the circuit in series so that whole of the current in the circuit passes through it. The deflection produced in the ammeter is a measure of the current.Since, however, the coil of the ammeter has some resistance, so on inserting it in the series of the circuit, the resistance of the circuit increases and the current in the circuit somewhat decreases. Therefore the current read by the ammeter is less than the actual current to be measured.

Hence it is necessary that the resistance of the ammeter be very small compared to other resistance in the circuit.

CONVERSION OF GALVANOMETER TO AMMETER

A galvanometer as such cannot be used as an ammeter because it has appreciable resistance and it can measure only a limited current corresponding to the maximum deflection on its scale.

An ammeter is made by connecting a low resistance S in parallel with a pivoted-type moving-coil galvanometer G. S is knows as "shunt". Its value depends upon the range of the required ammeter and can be calculated as follows:

Let G be the resistance of the coil of the galvanometer and ig be the current which, on passing through the galvanometer produces full-scale deflection. if i is maximum current to be measured, then a part ig of the current i should pass through the galvanometr G and the rest (i-ig) through the "shunt" S. Since G and S are parallel, the potential difference across them will be same :

ig * G =(i-ig) * S

ig/i = S/(S+G)

S=(i/(i-ig))*G

Resistance of ammeter:

1/R= 1/G + 1/S

R= GS/(G+S).

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MAGNETISM AND SOME DETAILS

Sailors used magnetic compasses to find their way on the world's oceans at least 1000 years ago, but the true nature of magnetism puzzled people for many centuries. Magnetism is an invisible force that comes from objects called magnets. The region around a magnet in which its magnetism acts is called a magnetic field.

Magnetic Earth

The earth itself act like a giant bar magnet, with a magnetic field and two magnetic poles. These poles are found near the earth's geographical north and south poles. The earth's magnetism is probably caused by the movement of  molten iron at the earth's core.

The earth's magnetic field stretches more than 60,000km (37,000 miles) out into space. In addition to affecting objects on the planet's surface, the earth's magnetism also affects electrically charged particles such as electrons and protons emitted by the sun. The other planets in the solar system also have magnetic fields, as does the sun itself.

Aurora:

The earth's magnetic poles pull electrically charged particles from the sun into the atmosphere. As the particles strike atoms or molecules in the air, coloured light is emmited in a dazzling display called an aurora.

Magnetic materials

When placed within a magnet's force field, some materials turn into magnets themselves-either briefly or permanently. These materials are said to be magnetic.

Inside a magnetic material, there are tine regions of magnetism called domains, all pointing in different directions. Their effects cancel out, so there is no overall magnetism.

In a magnet, the domains all point the same way. Their effects combine to give a strong magnetism.

Placing a bar magnet near a magnetic material causes the material's domains to line up and point in the same direction, turning it into a magnet. This is magnetic induction. This effect is usually temporary, but some material such as steel, stay permanently magnetized.

Magnetic forces

When two magnets are placed pole to pole, a force act between them. Different poles (a north and a south) pull each other together. This is attraction. Similar poles (two north or two south) push (repel) each other apart. This is repulsion.

SHUNT

 A galvanometer is used in electrical circuits to detect current and in experiments to determine the null point.

If somehow heavy current happens to flow into the coil of galvanometer, then due to very large deflection the pointer of the galvanometer may strike the 'stop pin' and be broken, or the coil of galvanometer may burn due to excessive heat produced. To save the galvanometer from these possible damages, a thick wire or a strip of copper is connected in parallel with its coil. It is called shunt. Its resistance is very small compared to the resistance of the coil. Therefore, most of the part of the current goes through the shunt and only a very small part goes through the coil. Hence there are no chances of the burning of the coil or the breaking of the pointer.

Shunted-galvanometer is very useful for determining the null-points in meter-bridge and potentiometer experiments. First, the approximate position of the null-point is determined by using the shunted galvanometer. At this stage the current in the circuit is very feeble. Now the shunt is removed from the galvanometer so that full current goes through the galvanometer and accurate position of the null-point is determined.