# Galvanometers ammeters and voltmeters

## Use of moving-iron instruments for measuring a.c.

Use of moving-iron instruments for measuring a.c. The main advantage of moving-iron instruments is that they can be used with alternating current (a. c.) as well as direct current (d.c.). Direct current flows in one direction only, while alternating current flows backwards and forwards many times a second. For example, a.c. from the electric supply mains has a …

## Moving-iron instruments

Moving-iron instruments Apart from the moving-coil instruments, there is another important class of meters known as moving-iron instruments. These are of two types, one based on repulsion and the other on attraction. The repulsion moving-iron instrument consists of a coil inside which are two softiron bars, one fixed and the other attached to a pivoted pointer. A hairspring …

## The Multimeter

The Multimeter Fig. 39.5 shows a galvanometer known as a universaL indicator. Basically, it is a microammeter with a range of 0-100 IlA. Its resistance in 1000 n, so it can also be used as a voltmeter with a full-scale deflection of In the picture, however, it is shown fitted with a multiplier which extends its range from 0 …

## Use of a multiplier to convert a milliammeter into a voltmeter

Use of a multiplier to convert a milliammeter into a voltmeter We have just seen that, provided we know its resistance, a milliammeter may be used as a millivoltmeter. Larger potential differences may be measured by placing a high resistance or multiplier in series with the milliammeter (Fig. 39.4). Since the total p.d. across the instrument and …

## Milliammeter used as a millivoltmeter

Milliammeter used as a millivoltmeter The milliammeter described on the preceding page has a full-scale deflection of 15 mA and a resistance of 5 n. It follows that the potential difference across it when it is giving a full-scale reading is given by p.d. = current x resistance = 0.015 x 5 = 0.075 V or 75 …

## Construction of an ammeter. Use of shunts

Construction of an ammeter. Use of shunts Moving-coil instruments are not made to take currents of more than a few milliamperes. If designed for larger currents the coil would have to be wound with much thicker wire, and the resultant instrument would be both clumsy and expensive. Fortunately there is an easy way out of the difficulty. When …

## Ammeters and voltmeters

Ammeters and voltmeters Ammeters and voltmeters are electric measuring instruments fitted with scales which have been graduated respectively to read current in amperes and potential difference in volts. The correct method of connecting an ammeter and a voltmeter in a circuit is shown on page 409. An ammeter is always placed in series with the resistance or other …

## Sensitivity of a moving-coil galvanometer

Sensitivity of a moving-coil galvanometer The deflecting couple on the coil is equal to the force on a vertical side multiplied by the width of the coil (see page 64). For a given current, the force on a vertical side will be proportional to its length. Hence the deflecting couple will be proportional to (length x width) …

## The moving-coil galvanometer

The moving-coil galvanometer The construction of a moving-coil galvanometer, calibrated for use as a milliammeter, is shown in Fig. 39. A rectangular coil carrying a pointer is pivoted on jewelled bearings and free to move in the annular (= ring-like) space between a soft-iron cylinder and the cylindrical pole faces of a strong magnet. Current is led into and …

## Galvanometers, ammeters and voltmeters

Galvanometers, ammeters and voltmeters The simple galvanometer or current-indicating instrument designed by Schweigger has already been described on page 419. Instruments of the moving-magnet type with various modifications to make them more. sensitive were in general use during the nineteenth century. They did, however, have certain disadvantages. Not only were they affected by stray magnetic fields set up by …