Month: May 2018

Alternating current flows periodically first in one direction and then in the opposite direction. One direction is called a positive alternation and the other direction is called a negative alternation. A complete positive and negative alternation is called one cycle. The number of complete cycles that occur each second is the frequency and is designated in hertz, abbreviated Hz. Therefore, if one complete cycle occurs per second, the frequency is 1 Hz; if 5 cycles are completed per second, the frequency is 5 Hz, and so on.

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The Basic Meter

Meters, except for the few that operate on electromagnetic principles can only measure the amount of current flowing through them. However, they can be calibrated to indicate almost any electrical quantity. For example, you know that according to Ohm’s law, the current that flows through a meter is determined by the voltage applied to the meter and the resistance of the meter: I=        V/ R

Continue reading “Introduction to Electromagnetism”

There are two methods of calculating the values of multiplier resistors for a multirange voltmeter. In the first method, each multiplier is calculated the same as for a single-range voltmeter. Assume that you wish to extend the range of a 1-mA movement to measure 0- 10, 0- 100, and 0- 1000 volts, and you also want a 0- 1-V range. Since full-scale deflection equals 1 Von the 0- 1-V range (V = IM RM = 0.001 A x 1000 Q = I volt), no multiplier is needed. The total resistance (RT OT) needed to limit meter current (IM) to 1 mA on the 0- 10-V range is RrnT = V w vJ IM   =   10 V/ 0.001A=    10,000 Q

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Current tests are somewhat more difficult than voltage tests because although voltage tests are done while the circuit is energized, the test probes need only touch test points to get a voltage reading. Using the clamp-on ammeter is similarly easy since it needs simply to be clamped to the energized wire. With the in-line ammeter, though, a current test requires the power to be shut down, the circuit opened, the meter connected in place, the circuit closed again, and the power turned on to get a reading. There are similar steps when the test is completed and the wiring must be reconnected. Care must be taken in handling disconnected, loose wiring, and the ammeter must be firmly wired into the circuit with good resistance-free connections. Be sure the meter is firmly supported.

Continue reading “Current Tests: Part 1”

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Analog meters are so called because they do not measure the electrical characteristics directly. They do their measuring indirectly by measuring the effect of what is being measured. The effect and the amount of the effect are considered analogous to the original characteristic being measured. Usually, the current is the first characteristic tested and the effects of the current, either magnetic or thermal, are converted to movement with a deflected pointer. The greater the current being measured, the greater is the magnetic or thermal effect, and the greater the pointer deflection. The movement or distance of the pointer action, then, is analogous to the amount of current being measured.

Continue reading “Analog Meters Introduction”