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It was mentioned that a good voltmeter will disturb the circuit under test as little as possible, and this requires that the meter have a high internal resistance. Besides the electrostatic type voltmeter, there is another way to get an extremely high internal resistance. This is to sample a tiny, tiny current, far too small for any meter to directly indicate, and then amplify this current so that a meter will show it. When a miniscule amount of current is drawn from a circuit, the equivalent resistance is always extremely high. The most effective way to accomplish the amplification, while making sure that the current drawn really is tiny, is to use either a vacuum tube or a field-effect transistor (FET). You needn t worry about how such amplifiers work right now; that subject will come much later in this book. A voltmeter that uses a vacuum tube amplifier to minimize the current drain is known as a vacuum-tube voltmeter (VTVM). If an FET is used, the meter is called a FET voltmeter (FETVM). Either of these devices provide an extremely high input resistance along with good sensitivity and amplification. And they allow measurement of lower voltages, in general, than electrostatic voltmeters.
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The measurement of electrical power requires that voltage and current both be measured simultaneously. Remember that power is the product of the voltage and current. That is, watts (P) equals volts (E) times amperes (I), written as P EI. In fact, watts are sometimes called volt-amperes in a dc circuit. You might think that you can just connect a voltmeter in parallel with a circuit, thereby getting a reading of the voltage across it, and also hook up an ammeter in series to get a reading of the current through the circuit, and then multiply volts times amperes to get watts consumed by the circuit. And in fact, for practically all dc circuits, this is an excellent way to measure power (Fig. 3-10). Quite often, however, it s simpler than that. In many cases, the voltage from the power supply is constant and predictable. Utility power is a good example. The effective voltage is always very close to 117 V. Although it s ac, and not dc, power can be measured in the same way as with dc: by means of an ammeter connected in series with the circuit, and calibrated so that the multiplication (times 117) has already been done. Then, rather than 1 A, the meter would show a reading of 117 W, because P EI 117 1 117 W. If the meter reading were 300 W, the current would be 300/117 2.56 A. An electric iron might consume 1000 W, or a current of 1000/117 8.55 A. And a large heating unit might gobble up 2000 W, requiring a current of 2000/117 17. 1 A. This might blow a fuse or breaker, since these devices are often rated for only 15 A. You ve probably had an experience where you hooked up too many appliances to a single circuit, blowing the fuse or breaker. The reason was that the appliances, combined, drew too
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Watt-hour meters 55
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3-10 Power can be measured with a voltmeter and an ammeter.
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much current for the house wiring to safely handle, and the fuse or breaker, detecting the excess current, opened the circuit. Specialized wattmeters are necessary for the measurement of radio-frequency (RF) power, or for peak audio power in a high-fidelity amplifier, or for certain other specialized applications. But almost all of these meters, whatever the associated circuitry, use simple ammeters as their indicating devices.
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The utility company is not too interested in how much power you re using with one appliance, or even how much power a single household is drawing, at any given time. By far the greater concern is the total energy that is used over a day, a week, a month or a year. Electrical energy is measured in watt hours, or, more commonly for utility purposes, in kilowatt hours (kWh). The device that indicates this is the watt-hour meter or kilowatt-hour meter. The most often-used means of measuring electrical energy is by using a small electric motor device, whose speed depends on the current, and thereby on the power at a constant voltage. The number of turns of the motor shaft, in a given length of time, is directly proportional to the number of kilowatt hours consumed. The motor is placed at the point where the utility wires enter the house, apartment or building. This is usually at a point where the voltage is 234 V. This is split into some circuits with 234 V, for heavy-duty appliances such as the oven, washer and dryer, and the general household fines for lamps, clock radios and, television sets. You ve surely seen the little disk in the utility meter going around and around, sometimes fast, other times slowly. Its speed depends on the power you re using. The total number of turns of this little disk, every month, determines the size of the bill you will get as a function also, of course, of the cost per kilowatt hour for electricity. Kilowatt-hour meters count the number of disk turns by means of geared, rotary drums or pointers. The drum type meter gives a direct digital readout. The pointer type has several scales calibrated from 0 to 9 in circles, some going clockwise and others going counterclockwise.
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56 Measuring devices Reading a pointer type utility meter is a little tricky, because you must think in whatever direction (clockwise or counterclockwise) the scale goes. An example of a pointer type utility meter is shown in Fig. 3-11. Read from left to right. For each little meter, take down the number that the pointer has most recently passed. Write down the rest as you go. The meter in the figure reads 3875 kWh. If you want to be really precise, you can say it reads 3875-1/2 kWh.
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