Text. Moving-coil ammeter and voltmeter

The most reliable and widely used ammeter or voltmeter has basically a moving-coil system.

The principle upon which both of these devices operate is essentially the same as that of the electric motor, differing from the motor, however, in the delicateness of their con­struction and the restrained motion of the rotating armature.

A coil of fine copper wire is so mounted between the two poles of a permanent magnet that its rotation is restrained by a hairspring. The farther the coil is turned from its equi­librium or zero position the greater is the restoring force. To this coil is fastened a long pointer at the end of which is a fixed scale reading amperes if it is an ammeter or volts if it is a voltmeter. Upon increasing the current through the moving coil of an ammeter or voltmeter the resultant magnetic field between coil and magnet is distorted more and more. The resulting increase in force therefore turns the coil through a greater and greater angle, reaching a point where it is just balanced by the restoring force of the hairspring.

Whenever an ammeter or voltmeter is connected to a circuit to measure electric current or potential difference, the ammeter must be connected in series and the voltmeter in parallel. As illustrated in Fig. 10, the ammeter is so con­nected that all of the electric current passes through it.

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To prevent a change in the electric current when making such an insertion, all ammeters must have a low resistance. Hence, most ammeters have a low resistance wire, called a shunt, connected across the armature coil.

A voltmeter, on the other hand, is connected across that part of the circuit for which a measurement of the potential difference is required. The potential difference between the ends of the resistance R₁ being wanted, the voltmeter is connected as shown. Should the potential difference across R2 be desired, the voltmeter connections would be made at С and D, whereas if the potential difference maintained by the battery were desired, they would be made at A and B, In order that the connection of a voltmeter to a circuit does not change the electric current in the circuit, the voltmeter must have a high resistance. If the armature coil does not have a large resistance of its own, additional resistance is added in series.

Listen and read words and word combinations to be remem­bered:

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1

Models

1. If the armature coil does not. have a large resistance of its own, additional resistance is added in series.

If he studies electricity he knows the construction of an ammeter. If he studied this material, he solved this problem.

2. If the potential difference maintained by the battery were desired, the voltmeter connections would be made at A and B.

If I had a book about ammeters and voltmeters, I should give it to you. If he were at the laboratory he would meas­ure the electric current. Had he known the rule, he would not have made so many mistakes.