Kinds of circuits

Circuits can be divided into four classes: series, parallel, combination of series-parallel, and network.

Series circuits are those having only one closed path for the flow of electricity. All the elements, or devices which make up the circuit are connected in tandem, one after the other, so that the end of one is connected to the beginning of the other; or, in other words, the positive terminal of one is con­nected to the negative terminal of another. If the series circuit is opened anywhere, the current will not flow through the circuit.

A parallel circuit is one divided into two or more branches, each branch carrying part of the current. Another way of say­ing the same thing is that all the elements or devices are con­nected so that one half of the terminals are fastened to a com­mon point or a common conductor, and the other half are fastened to another common point, or another conductor.

MEASURING DEVICES

Ammeters and Voltmeters. — 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 would 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. C. 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.