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 currents of several amperes have to be measured a low resistance which by-passes the greater part of the current is placed in parallel with a milliammeter or a microammeter, and then only a small known fraction of the total current passes through the meter itself.A resistance used in this way is called a shunt.
Suppose, for example, a milliammeter of resistance 5 nand full-scale deflection 15 mA is to be used for the purpose of measuring currents to 1.5 A. It would be necessary to use a shunt which passed (1.5 – 0.015) = 1.485 A while the meter carried only 0.015 A (Fig. 39.3).We shall now show how to calculate the value of the shunt required. Let the resistance of the shunt be R. Since the shunt and the meter are in parallel, there will be the same potential difference V across each. We can use this fact to obtain two equations from which R may be calculated.
A shunt of this value would take the form of a short piece of fairly thick manganin wire or strip carefully adjusted to have the exact resistance required. Manganin is chosen for shunts, since its resistance does not alter when its temperature is raised by the passage of current.
When the shunt is in use the scale readings of the instrument are divided by 10 and called amperes instead of milliamperes. An ordinary ammeter with a range of 1.5 A will, of course, have its shunt connected up inside the case of the instrument.