567 STANDARD ELECTRICAL DICTIONARY.

Voltametric Law.

The law on which voltameters are based. The amount of chemical decomposition produced by an electric current in a given electrolyte is proportional to the quant.i.ty of electricity pa.s.sed through the solution.

Fig. 344. VOLTA'S FUNDAMENTAL EXPERIMENT.

Volta's Fundamental Experiment.

The moistened finger is placed on the upper plate of a condensing or electrophorous electroscope. The other hand holds a plate of zinc z, soldered to a plate of copper c. The lower plate is touched with the copper. On removing the cover the gold leaves l diverge and with negative electricity. Hence zinc is supposed to be positively electrified when in contact with copper. The experiment is used to demonstrate the contact theory of electricity.

568 STANDARD ELECTRICAL DICTIONARY.

Volta's Law of Galvanic Action.

The electro-motive force between any two metals in an electro-chemical series (see Electro-Chemical Series) is equal to the sum of the electro-motive forces between all the intervening metals.

Volta's Law of Thermo-electricity.

In a compound circuit, consisting of a number of different metals, all points of which are at the same temperature, there is no current.

Volt, B. A.

The volt based on the B. A. ohm. It is equal to .9889 legal volt.

Volt, Congress.

The volt based upon the congress or legal ohm; the legal volt.

Volt-coulomb.

The unit of electric work; the watt-second; it is equivalent to 1.0E7 ergs.

.24068 gram degree C. (calorie) .737337 foot lbs., .00134 horse power seconds.

Volt Indicator.

A form of easily read voltameter for use in electric light stations and for similar work.

Volt, Legal.

The legal volt based upon the legal ohm. It is equal to 1.00112 B. A.

volt.

Voltmeter.

An instrument for determining the potential difference of any two points.

In many cases it is a calibrated galvanometer wound with a coil of high resistance. The object to be attained is that it shall receive only an insignificant portion of current and that such portion shall suffice to actuate it. If connected in parallel with any portion of a circuit, it should not noticeably diminish its resistance.

The divisions into which ammeters range themselves answer for voltmeters. In practice the same construction is adopted for both. The different definitions of ammeters in disclosing the general lines of these instruments are in general applicable to voltmeters, except that the wire winding of the coils must be of thin wire of great length. The definitions of ammeters may be consulted with the above understanding for voltmeters.

In the use made of voltmeters there is a distinction from ammeters. An ammeter is a current measurer and all the current measured must be pa.s.sed through it. But while a voltmeter is in fact a current measurer, it is so graduated and so used that it gives in its readings the difference of potential existing between two places on a circuit, and while measuring the current pa.s.sing through its own coils, it is by calibration made to give not the current intensity, but the electro-motive force producing such current.

In use it may be connected to two terminals of an open circuit, when as it only permits an inconsiderable current to pa.s.s, it indicates the potential difference existing between such points on open circuit. Or it may be connected to any two parts of a closed circuit. Owing to its high resistance, although it is in parallel with the intervening portion of the circuit, as it is often connected in practice, it is without any appreciable effect upon the current. It will then indicate the potential difference existing between the two points.

569 STANDARD ELECTRICAL DICTIONARY.

Voltmeter, Battery.

A voltmeter for use in running batteries. In one form (Wirt's) it is constructed for a low range of voltage, reading up to two and a half volts and having exactly one ohm resistance, thus giving the battery some work to do.

Voltmeter, Cardew.

A voltmeter in which the current pa.s.sing through its conductor heats such conductor, causing it to expand. Its expansion is caused to move an index needle. By calibration the movements of the needle are made to correspond to the potential differences producing the actuating currents through it. The magnetic action of the current plays no part in its operation. It is the invention of Capt. Cardew, R. E.

The construction of the instrument in one of its most recent forms is shown in the cut. On each side of the drum-like case of the instrument are the binding screws. These connect with the blocks m and n. To these the fine wire conductor is connected and is carried down and up over the two pulleys seen at the lowest extremity, its centre being attached to c. From c a wire is carried to the drum p, shown on an enlarged scale on the left of the cut. A second wire from the same drum or pulley connects to the spring S. The winding of the two wires is shown in the separate figure of c, where it is seen that they are screwed fast to the periphery of the little drum, and are virtually continuations of each other. By the screw A the tension of the spring S is adjusted.

On the shaft of the little drum p is a pinion, which works into the teeth of the cog-wheel r. The shaft of r is extended through the dial of the instrument, and carries an index. The dial is marked off for volts; g g and h h are standards for carrying the pulleys.

570 STANDARD ELECTRICAL DICTIONARY.

The action of the instrument is as follows. The current pa.s.sing through the wire heats it. This current by Ohm's law is proportional to the electro-motive force between the terminals. As it is heated it expands and as it cools contracts, definite expanding and contracting corresponding to definite potential differences. As the wire expands and contracts the block or pin c moves back and forth, thus turning the drum p and cogwheel r one way or permitting it to turn the other way under the pull of the spring S.

Fig. 345. CARDEW VOLTMETER.

In this construction for a given expansion of the wire the piece c only moves one half as much. The advantage of using a wire twice as long as would be required for the same degree of movement were the full expansion utilized is that a very thin wire can be employed. Such a wire heats and cools more readily, and hence the instrument reaches its reading more quickly or is more deadbeat, if we borrow a phraseology properly applicable only to instruments with oscillating indexes.

In the most recent instruments about thirteen feet of wire .0025 inch in diameter, and made of platinum-silver alloy is used.

571 STANDARD ELECTRICAL DICTIONARY.

If the potential difference to be measured lies between 30 and 120 volts the wire as described suffices. But to extend the range of the instrument a resistance in series is required. If such resistance is double that of the instrument wire, and remains double whether the latter is hot or cold the readings on the scale will correspond to exactly twice the number of volts. This is brought about in some instruments by the introduction in series of a duplicate wire, precisely similar to the other wire, and like it, carried around pulleys and kept stretched by a spring.

[Transcriber's note: If the series resistance is twice that of the voltmeter, the indicated voltage will be ONE THIRD of the total voltage.]

Thus whatever ratio of resistance exists between the two wires cold, it is always the same at any temperature, as they both increase in temperature at exactly the same rate. Tubes are provided to enclose the stretched wires and pulleys, which tubes are blackened.

The voltmeter is unaffected by magnetic fields, and, as its self-induction is very slight, it is much used for alternating currents.

The tubes containing the wire may be three feet long.