Resistance Network

TM 11-6625-298-14

Figure 5-3. Ohmmeter ZM-21/U (and some earlier model ZM-21A/U). schematic diagram.

Figure 5-4. Ohmmeter ZM-21A/U and ZM-21B/U, schematic diagram.

5-4. Resistance Network

5-5. Meter Movement

The resistance network of Ohmmeter ZM-21B/U

a. The meter movement is shown in figure 5-5.

consists of one 90,000 ohm resistor, R1, and two

The indicating instrument consists of two coils,

250,000-ohnm resistors R2 and R3 (fig. 5-4). The

potential coil L2, and current coil L1. These coils,

two 250,000-ohm resistors are connected in series

mounted on a common axis and connected so that

between one side of meter potential coil L2 of the

they oppose each other in a definite ratio, rotate

ohmmeter and the positive side of the generator

between two pole pieces in a magnetic field. One

force tends to move the indicating pointer toward

(through the controller). Resistor R1 is connected

between one side of meter current coil L1 of the

ZERO on the ohmmeter scale; the other force

ohmmeter and the LINE terminal and acts as a

tends to move the pointer toward INFINITY.

protective resistance to guard against damage

b. Current flows to the indicator through

caused by excessive current flowing in coil L1.

flexible ligaments of practically zero torque which

5-3