# 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 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.