3-10The formula for determining the resistance of the shunt is given by Rs = I_{m} /Is times R_{m}, where R_{s }isthe shunt resistance in ohms; I_{m} is the meter current at full-scale deflection; Is is the shunt current at full-scale deflection; and R_{m} is the resistance of the meter coil. If the values given in the previous example areused in this equation, it will yield 5.55 ohms, the value previously calculated.SWITCHING SHUNT VALUES.—Various values of shunt resistance can be used, by means of asuitable switching arrangement, to increase the number of current ranges that can be covered by themeter. Two switching arrangements are shown in figure 3-5. View A is the simpler of the twoarrangements when a number of shunts are used to calculate the values of the shunt resistors. However, ithas two disadvantages:Figure 3-5.—Ways of connecting internal meter shunts.1.When the switch is moved from one shunt resistor to another, the shunt is momentarily removedfrom the meter. The line current then flows through the meter coil. Even a momentary surge ofcurrent could easily damage the coil.2.The contact resistance (resistance between the blades of the switch when they are in contact) is inseries with the shunt, but not with the meter coil. In shunts that must pass high currents, thiscontact resistance becomes an appreciable part of the total shunt resistance. Because the contactresistance is of a variable nature, the ammeter indication may not be accurate.The generally preferred method of range switching is shown in (figure 3-5, view B). Although onlytwo ranges are shown, as many ranges as needed can be used. In this type of circuit, the contact resistanceof the range-selector switch is external to the shunt and meter in each range position. The contactresistance in this case has no effect on the accuracy of the current measurement.Ammeter ConnectionsWhen you are using ammeters, a primary rule of safety is that such current-measuring instrumentsmust always be connected in series with a circuit, never in parallel with it. When an ammeter isconnected across a constant-potential source of appreciable voltage, the low internal resistance of themeter bypasses the circuit resistance. This results in the application of the source voltage (or a good