a. For this discussion, we can use the same meter as used for the ammeter.

The meter sensitivity is 1 milliampere and coil resistance is 75 ohms. Assume we

need to determine the correct multipliers for ranges of 0 to 10 volts, 0 to 100

volts, and 0 to 500 volts (fig 1-15).

Since 1 milliampere causes a full-scale

deflection, the total resistance of the meter (R1+75) must be such that the voltage

across it is 10 volts (0-to 10-volt range) when 1 milliampere current is flowing.

Thus:

Figure 1-15.

Determining multipliers

b. The total resistance is 10,000 ohms and the resistance of the multiplier

(R1) is 9925 ohms. The resistance of the other two multipliers (R2 and R3) can be

determined in a similar manner.

c. A term which you frequently encounter while troubleshooting television

systems is voltmeter sensitivity. The sensitivity of a voltmeter is expressed in

ohms per volt and is determined by dividing the resistance of the meter and the

multiplier by the full-scale reading in volts. It is just another way of stating

what current can cause full-scale deflection.

d. A voltmeter should have very high resistance so it draws very little

current and affects the circuit as little as possible during voltage measurements.

Sensitivity, therefore, is an indication of the measuring quality of a voltmeter.

Generally, a meter with a 100-ohms-per-volt sensitivity is satisfactory. However,

for good accuracy in circuits with high resistance, you must use a meter with a

sensitivity of 20,000 (or more) ohms per volt.

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