II. Write out from the text the sentences with verbs in the form of the Participle or the Gerund or the Subjunctive mood. Measuring devices
Ammeters and Volt meters.
- Ammeters measure the current flowing in a circuit and normally have scales which are graduated or calibrated in amperes, milliamperes or microamperes.
Voltmeters are used to measure the potential difference between two points in a circuit. The calibration of voltmeters is usually in volts, millivolts or microvolts.
The main difference between the two instruments of the same type or design is in the
resistance of the operating coil, identical moving units may be used for either meter. An ammeter
is connected in the positive or negative lead in series with a circuit and, therefore, must have a
low resistance coil, otherwise the readings will be incorrect as the coil would absorb an
appreciable amount of power.
A voltmeter is connected in parallel across the points of a circuit where the difference of potential is to be measured. The resistance of the operating coil must, in this instance, be as high as possible, to limit the amount of current consumed by it, or else a drop in potential due to the meter would occur and the pointer indication would not represent the true potential difference across the circuit.
Wattmeters .- The measurement of the power in a D. С circuit at any instant can be achieved by means of an ammeter and voltmeter as the power in watts is the product of the current and the voltage. With A.C. circuits, however, the instantaneous values are always changing. To measure A.C. power correctly, therefore, it is necessary to use the third instrument to measure the phase difference. The normal practice, however, is to combine these three instruments in one which will give a direct reading of power in watts.
