Voltmeter

“ Such a galvanometer, fitted with a carefully graduated scale, in this way will indicate the number of volts, and it is called a voltmeter. If we have a galvanometer with a short coil of very thick wire, the resistance put in the way of the current is so small that it may be left out of account, and by means of a graduated scale the number of amperes may be shown ”

“ That is, the alternating current will have about 11 per cent. more heating power than the direct current which is of the same average strength.
If an alternating current voltmeter be placed upon a circuit in which the volts range from +100 to -100, it will read 70.7 volts, although the arithmetical average, irrespective of + or-sign, is only 63.7 volts. If the voltmeter be connected to a direct current circuit, the pressure necessary to give the same reading would be 70.7 volts. ”

“ In either case, the product of the ammeter reading in amperes and the voltmeter reading in volts will give the power taken by the heater in watts, assuming the heater winding to be noninductive. If the heater winding is not noninductive, then the current and the electrical pressure will no longer be in phase when the device is operating on an alternating-current circuit, and a wattmeter must be used. Practically all heating elements are wound noninductively so that the power may be measured by means of an ammeter and voltmeter. ”

“ Fig. 75 shows a voltmeter V, and ammeter A, so placed in the circuit that readings can be taken. D represents a dynamo. A is placed so that the whole current passes through it, while V is placed between the main wires to measure the difference in potential. The product of the two readings in volts and amperes gives the number of watts. ”

“ Supposing the same change ratio it is evident that if both remain unloaded the voltmeter will indicate the same pressure. A gradually increasing lamp load up to the limit of the transformer capacity, will be attended by a drop in pressure at the terminals. This drop can be read as the difference of the voltmeter indications, and when expressed in per cent. of secondary voltage stands for "regulation." Remarks: The auxiliary transformer is necessary in order to make sure of the high tension line voltage. A large transformer under test may cause primary drop in taking power. ”

“ One and the same instrument may be used as an ammeter or as a voltmeter, by providing it with shunts for use as an ammeter, and series resistances for use as a voltmeter. If the current to be measured is large, the shunt will have a very low resistance compared with that of the instrument, so that only a small fraction of the total current passes through the instrument. Similarly, when a large P.D. is being measured, the series resistance will have a very high value, so that the current [192] through the instrument may not exceed that which gives full-scale reading. ”

“ As the frame has the same linear expansion as the wire, external changes of the temperature will not affect their relative length, but if the fine wire is heated by the passage of an electric current, its expansion will move the indicating needle over the scale, the motion being multiplied by the gear. In the Hartmann and Braun form of hot-wire voltmeter, the fine wire is fixed between two supports, and the expansion produced when a current is passed through it causes the wire to sag down, the sag being multiplied by a gear and made to move an indicating needle over a scale. ”

“ A compensator proper consists of a variable resistance and a variable inductance, and sometimes a current transformer. In wiring, the voltmeter, instead of being connected directly across the secondaries of a pressure transformer, has inserted in series with it, portions of the resistance and inductance of the compensator. These are so connected that the drop in pressure across them will be combined with that of the pressure transformer, so that the voltmeter reading indicates the pressure at the center of distribution or end of the line. ”

“ This voltmeter has a 2.5 and a 25 volt scale, which makes it convenient for battery work. It also gives readings of a .2 and 2.0 to the left of the zero, and special scale markings to facilitate the making of Cadmium tests as described on page 174. As for the ammeter, if a motor-generator set, Tungar Rectifier or a charging-rheostat is used, the ammeter is always furnished with the set. If a lamp bank is used, a switchboard type meter reading to about 25 amperes is suitable. ”

“ Electromagnetic voltmeters consist of a coil of fine wire connected to the terminals of the instrument, and the current produced in that wire by a difference of potential between the terminals creates a magnetic field proportional at any point to the strength of the current. ”

“ Without a voltmeter, the positive terminal of the charging circuit can be determined by attaching a piece of clean lead to each wire which is to be connected to the battery, and immersing them, without touching each other, in a glass or other insulating vessel containing water to which is added a drop or two of sulphuric acid. After the current has passed through the circuit for a short time, the positive lead will commence to discolor, and, if left long enough, will turn brown. ”

“ The electrostatic voltmeter mentioned above is the only instrument which is operated by a P.D. instead of a current. In it a moving vane is attracted into a fixed pair of quadrants; or a set of vanes is attracted into a set of quadrant cells.
The majority of electricity meters are of the motor type, i.e. a disc is driven by motor action at a speed which is proportional to the power passing through the meter. The disc spindle engages with gearing which drives the pointers on a set of dials recording the energy units. ”

“ The current may be regulated by altering the distance between the plates or by varying the strength of the solution. As the discharge progresses, the voltage will gradually decrease and it should be frequently read at the battery terminals. When it shows a sudden drop, the voltage of each cell should be read with a low reading voltmeter.
While the readings are being taken, the discharge rate should be kept constant and the discharge continued until the majority of the cells read 1.70 volts; those reading less should be noted. ”

“ Like the corresponding ammeters, they have the great advantage that the scales are equidivisional and that there is no dead part in the scale, whereas both the electrostatic and electrothermal voltmeters, above described, labour under the disadvantage that the scale divisions are not equal but increase with rise of voltages, hence there is generally a portion of the scale near the zero point where the divisions are so close as to be useless for reading purposes and are therefore omitted. ”

“ The apparatus is calibrated by passing a known constant current through the resistance.
Fig. 2,590.—Four part commutator method of wave form measurement. The contact device consists of two slip rings and a four part commutator. One slip ring is connected to one terminal of the source, the other to the voltmeter, and the commutator to the condenser. By adjusting R when a known direct current pressure is impressed across the terminals, the voltmeter can be rendered direct reading. ”

“ In the secondary circuit of the transformer is placed another watt meter of a similar kind, or, if the load on the secondary circuit is non-inductive, the secondary voltage and the secondary current can be measured with a proper alternating current ammeter, A2, and voltmeter, V2, and the product of these readings taken as a measure of the power given out by the transformer. The ratio of the powers, namely, that given out in the external secondary circuit and that taken in by the primary circuit, is the efficiency of the transformer. ”

“ Power Loss in a Transformer.—When the voltage is "stepped up" in a transformer, do we gain power? To answer this question we must remember that electric power does not depend on voltage alone but on the product of e.m.f. and current intensity. (See Art. 291.) By tests with a.-c. voltmeters and ammeters, we find that when the secondary e.m.f. is greater than the primary e.m.f., the secondary current intensity is less than that in the primary. ”

“ Fig. 2,887.—Alternator excitation or magnetization curve test. The object of this test is to determine the change of the armature voltage due to the variation of the field current when the external circuit is kept open. As here shown, the field circuit is connected with an ammeter and an adjustable resistance in series with a direct current source of supply. The adjustable resistance is varied, and readings of the voltmeter across the armature, and of the ammeter, are recorded. The speed of the generator must be kept constant, preferably at the speed which is given on the name plate. ”

“ The fuel cell stack meter (used as a battery ammeter on spacecraft 6) with associated selector switch, provides a display of individual main battery test current with the selector in BT position and a particular MAIN BATIERIES switch in TEST position. With the selector switch in 1A, 1B, 1C or 2A, 2B, 2C positions, the ammeter displays 50 percent of the applicable adapter module battery current. The meter has a 0-20 ampere scale.
VOLTMETER On spacecraft 6 the voltmeter, used in conjunction with a selector switch, displays main bus, common control bus and squib bus voltage. ”

“ The polarity of the direct-current terminals depends upon the position the armature happens to be in when the alternating current is applied to it and is very apt to come in wrong when the machine is started. It is therefore necessary to have a polarity indicating voltmeter in the circuit and to watch it when starting the machine. If the polarity is wrong, the switch must be opened and in a moment thrown [203] in again; and if still wrong, this process must be repeated until the polarity comes right. Each arc lamp fed from a converter must be equipped with resistance. ”

“ In external appearance they are not distinguishable from ammeters.
FIG. 236.—A voltmeter.
The unit of electromotive force is called the volt. The voltage of a dry cell is approximately 1.5 volts, and the voltage of a voltaic cell and of a gravity cell is approximately 1 volt.
316. Current, Voltage, Resistance. We learned in Section 287 that the strength of a current increases when the electromotive force increases, and diminishes when the electromotive [345] force diminishes. Later, in Section 288, we learned that the strength of the current decreases as the resistance in circuit increases. ”

“ Never allow the storage battery to run down to a voltage lower than 1.15 per cell. This reading is taken from the voltmeter supplied with the plant.
Storage batteries should be tested by a hydrometer for the specific gravity of the electrolyte or liquids in them. Instructions for this and for correcting the specific gravity accompany the plant. Take care to preserve them.
Dynamos for home use are almost automatic. Run the dynamo to renew the batteries when using electric irons or other devices calling for more current than the lighting fixtures. ”

“ There are methods of measuring electrical power by means of electrostatic voltmeters, or of quadrant electrometers adapted for the purpose, which when so employed may be called electrostatic wattmeters. ”

“ Self-registering voltmeters indicate at any moment the potential difference in every tank, and therefore give notice of short circuits occurring at any part of the installation. The chief differences between the commercial systems of refining lie in the arrangement of the baths, in the disposition and manner of supporting the electrodes in each, in the method of circulating the solution, and in the current-density employed. The various systems are often classed in two groups, known respectively as the Multiple and Series systems, depending upon the arrangement of the electrodes in each tank. ”

“ Its action depends upon the heating of a conductor by the current flowing through it, causing it to expand and move an index needle or pointer, the movements of which, by calibration, are made to correspond to the pressure differences producing the actuating currents.
Ques. What are the characteristics of hot wire instruments?
Ans. Voltmeters of this type are not affected by magnetic fields, and as their self-induction is small, they can be used on [549] either direct or alternating currents; but they possess certain serious defects: they consume more current than the other types ”

“ Suppose, for instance, I needed to supply fifty amperes at one hundred-volt pressure ten miles distant from the generator, and had a conductor the size of a trolley wire to bring the current. The resistance of the trolley wire is one ohm for every two miles, or five ohms. The drop in voltage is found by multiplying the amperes of current by the ohms of resistance. Ten miles from the central station, therefore, the drop on fifty amperes would be 50 × 5 = 250 volts. It would, therefore, be necessary to maintain a pressure of 350 volts at the generator to deliver the fifty amperes at 100 volts. ”

“ Finally, the watt meter, an instrument recently developed by his firm, was shown in operation. This consisted simply of a coil of thick conductor suspended by a torsion wire, and opposed laterally to a fixed coil of wire of high resistance. The current to be measured flowed through both coils in parallel circuit, the one representing its quantity expressible in amperes, and the other its potential expressible in volts. Their joint attractive action expressed therefore volt-amperes or watts, which were read off upon a scale of equal divisions. ”