ELECTRICAL MEASUREMENTS.

If water could be neither seen nor felt we should have to determine its volume and height of fall by the effects of the stream upon various sizes of waterwheels, the effect of steam pressure upon a piston, etc.

With the growth of electrical science many specially designed instruments for measurments of electricity have been placed upon the market; almost the first was

FIG. 44.

The Galvanometer, which is chiefly used to measure battery currents. A galvanometer must fulfil the essential condition that its readings shall really measure the strength of the current in some certain way. It should also be sufficiently sensitive for the currents that are to be measured to affect it. The galvanometer adapted for measuring very small currents (say a current of only one or two millionth parts of an ampère) will not be suitable for measuring very strong currents, such as are used in producing an electric light. Moreover, if the current to be measured

THE GALVANOMETER.

has already passed through a circuit of great resistance (as, for example, some miles of telegraph wire), a galvanometer

NOTE.-Strong currents must not be passed through very sensitive galvanometers, for, even if they are not spoiled, the deflections of the needle will be too large to give accurate measurements. In such cases the galvanometer is used with a shunt, or coil of wire arranged so that the greater part of the current shall flow through it, and pass the galvanometer by, only a small portion of the current actually traversing the coils of the instrument.

ELECTRICAL MEASUREMENTS.

whose coil is a short one, consisting only of a few turns of wire will be of no use.

The Galvaniscope is an instrument used for ascertaining whether a current is flowing or not. It is an indicator of currents where the movement of the needle shows the direction of the current, and indicates whether it is a strong or a weak When the value of the readings have been determined by experiment or calculation any galvaniscope becomes a galvanimeter.

one.

FIG. 45.

THE ELECTROSCOPE.

NOTE.-The Henly quadrant electroscope is sometimes employed as an indicator for large charges of electricity. It consists of a pith ball at the end of a light arm fixed on a pivot to an upright. When the whole is electrified the pith ball is repelled from the upright and flies out at an angle indicated on a graduated scale or quadrant behind it. The device shown in Fig. 45 illustrates the same principle of "electric repulsion."

ELECTRICAL MEASUREMENTS.

The Electroscope is an instrument for detecting whether a body is electrified or not and indicating also whether the electrification is positive or not, The earliest electroscope devised consists of a stiff straw balanced lightly upon a sharp point; a thin strip of brass or wood, or even a goose quill, balanced upon a sewing needle, will serve equally well. When an electrified body is held near the electroscope it is attracted and turned round, and will thus indicate the presence of electricity.

There are many forms of the electroscope, notable among which are: 1. The Pith Ball; 2. The Gold Leaf; 3. The Condensing; 4. Bennett's; 5. Bohenberger's. Care must be used not to confuse the idea of the electroscope, which only indi cates the presence, with the following:

The Electrometer is an instrument for use in measurement of potential difference by the attraction of bodies charged with static electricity.

The "quadrant electrometer" designed by Lord Kelvin is sometimes made very sensitive so as to measure as low as To volt, but it is a very delicate laboratory instrument. It is valuable for measuring differences of potential where no current is flowing or where current is undesirable.

The Mirror Galvanometer.-When a galvanometer of great delicacy is needed, the moving parts must be made very light and small. To watch the movements of a very small needle an index of some kind must be used, indeed, in the

ELECTRICAL MEASUREMENTS.

tangent galvanometer, it is usual to fasten to the short stout needle a delicate stiff pointer of aluminum. A far better method is to fasten to the needle a very light mirror of silvered glass, by means of which a beam of light can be reflected on to a scale, so that every slightest motion of the needle is magnified and made apparent. The mirror galvanometers devised by Sir W. Thomson for signalling through submarine cables, are admirable examples of this class of instrument.

Fig. 46 represents a form of portable galvanometer, suitable for ordinary testing.

ELECTRICAL MEASUREMENTS.

Measurement of electric pressure and current volume.

The foregoing described instruments belong rather to the experimental and testing departments of electrical science, but the introduction of the dynamo and motor, with their powerful currents have demanded other instruments of greater range and capacity. This has caused the development of instruments for measuring

The Volt.

The Ampere.

The Ohm.

The Volt is the practical unit of measurement of pressure. The Ampere is the practical unit of measurement of rate of flow.

The Ohm is the practical unit of measurement of resistance.

An Ammeter or ampere meter is a device for measuring the number of amperes which are passing through a current and showing the same by direct reading on a scale.

The ammeter is a commercial form of galvanometer in which the deflections (or twistings) of a magnetic needle are valued in amperes.

Ammeters are made in various forms based upon several different principles, among which are

1. Permanent-magnet ammeters.

2. Electro-magnet ammeters.

3. Spring ammeters.

4. Gravity ammeters.