Electrical Indicating and Test Instruments 1 Introduction 161

Clamp-on Meters

These are used for measuring circuit currents and voltages in a noninvasive manner that avoids having to break the circuit being measured. The meter clamps onto a current-carrying conductor, and the output reading is obtained by transformer action. The principle of operation is illustrated in Figure 7.5, where it can be seen that the clamp-on jaws of the instrument act as a transformer core and the current-carrying conductor acts as a primary winding. Current induced in the secondary winding is rectified and applied to a moving coil meter. Although it is

Figure 7.5. Schematic drawing of a clamp-on meter.

a very convenient instrument to use, the clamp-on meter has low sensitivity and the minimum current measurable is usually about 1 amp.

Analogue Multimeter

The analogue multimeter is now less common than its counterpart, the digital multimeter, but is still widely available. It is a multifunction instrument that can measure current and resistance, as well as d.c. and a.c. voltage signals. Basically, the instrument consists of a moving coil analogue meter with a switchable bridge rectifier to allow it to measure a.c. signals, as shown in Figure 7.6. A set of rotary switches allows the selection of various series and shunt resistors, which make the instrument capable of measuring both voltage and current over a number of ranges. An internal power source is also provided to allow it to measure resistances as well. While this instrument is very useful for giving an indication of voltage levels, the compromises in its design that enable it to measure so many different quantities necessarily mean that its accuracy is not as good as instruments that are purposely designed to measure just one quantity over a single measuring range.