Professor Tell, in Steam Engineering.

A voltmeter is an instrument that indicates the voltage, or electro-motive force, that acts in a circuit. Electro-motive force is the force that impels acurrent through the circuit - that pushes it along, so to speak. It is substantially equivalent to pressure in steam, and in fact is sometimes spoken of as electric pressure. Electro-motive force is measured in volts, and thus it has come to be common practice to speak of the electromotive force acting in a circuit as the voltage.

Voltmeters are only made in the form of indicating and recording instruments; no other form is necessary, because all we require to know is what the voltage may be at any instant, which we ascertain by looking at the dial of an indicating instrument; or what the voltage has been at every instant of time during a certain period, which we can find by inspecting the chart of a recording voltmeter.

Recording voltmeters are similar in construction to recording steam gages, and are provided with either a roll of paper or a circular disk, upon which a line is traced showing the voltage at all instants of time within a certain period. In this article we will discuss the indicating voltmeter only. The way in which a voltmeter is connected in the circuit is illustrated in Fig. 1, in which a and m represent the armature and field coils of a generator, and V the voltmeter. As will be seen, one of the wires from the instrument is connected with linep, and the other with line n; thus the voltmeter short-circuits the entire external circuit. If an ammeter were placed in this position, there would be an enormous rush of current through it, sufficient to stall the generator and probably do great damage if a circuit breaker were not connected in the circuit. With a voltmeter, however, the case is very different, the difference being due to the fact that while the resistance of the ammeter is very low, that of the voltmeter is very high. The ammeter is provided with a coil of very large wire consisting of only a few turns - in large instruments, only one turn. The voltmeter, on the other hand, has coils wound with many turns (running up into the thousands) of very fine wire. Owing to this difference in the resistance, while the current passing through an ammeter connected in the position shown in Fig. 1 would be very great,- only limited by the capacity of the generator, - the current passing through the voltmeter would be very small, so small as to be measured in thousandths of an ampere. The actual strength of the current that passes through a voltmeter depends upon the resistance of its coils and the voltage that acts to force a current through them. The resistance does not change, hence the only way in which the strength of the current can be changed is by varying the voltage; therefore the instrument will show variations in voltage if properly calibrated.

From the foregoing it will be seen that ammeters and voltmeters act in precisely the same manner, that is, by the variation in the strength of the current that passes through them. The only difference between the two instruments is that the ammeter is constructed so as to be traversed by very strong currents, and is calibrated so as to show the number of amperes that pass through it; while the voltmeter is constructed so as to be traversed by very weak currents, and is calibrated so as to indicate the voltage that forces the current through it.

Fig. 1.

Fig. 2.

If a voltmeter is connected as in Fig. 1, it will indicate the highest voltage acting upon the external circuit, and will show a greater number of volts than if connected across the circuit at any other point farther away from the generator. The reason for this difference is that it requires electro-motive force to drive a current through even the shortest length of conductor, and therefore the voltage left after passing through a portion of the circuit will be less than before passing through it.

In Fig. 1 the generator shown is of the simple shunt type, this being used so as to simplify the diagram. When a compound-wound generator is used, it is necessary to be more particular in connecting the instrument, for if it is connected directly with the armature terminals, it will show a greater number of volts than actually act upon the external circuit. This fact is illustrated in Fig. 2, in which a properly connected voltmeter is shown at V, while one that only spans the armature is shown at V. As can be seen at once, the current in wire n cannot reach the armature until it has passed through the field coil to', and, as already stated, a certain amount of electro-motive force is required to drive the current through to'; hence the voltage between the wires p and n cannot be as great as that between b b, so that instrument V will indicate higher than instrument V.

Fig. 3.

The connection of voltmeters, as in Fig. 2, is commonly used in testing generators. If an ammeter is connected in one of the b wires, then by dividing the volts indicated by V by the number of amperes shown in the ammeter, the resistance of the armature is obtained in ohms. In this way we can determine what the armature resistance is when it is heated by the current. If we note the difference between the volts indicated by the instrument at Fand the one at F', and divide this difference by the number of amperes indicated by an ammeter connected in wire n, we will obtain the resistance of the field coil to'. The ammeter connected in n will not show as many amperes as that connected in the b wire, for a portion of the current passing through the latter instrument goes through the shunt field coil to and does not enter wire n. The difference between the readings of the two ammeters will show the current that flows through the shunt coil m. If we divide the volts indicated by the instrument V by the amperes of current passing through to, - which is the difference between the readings of the two ammeters, - we will obtain the resistance, in ohms, of the coil m. From this it will be seen that by connecting two voltmeters, as in Fig. 2, and an ammeter in wire b and another one in wire n, we can measure the current flowing through the armature, through the series field coils m' and through the shunt coil to; and we can also measure the resistance of these three parts of the machine when they are traversed by the normal working currents. Such measurements are taken for determining the efficiency of a generator and likewise its regulating qualities.